Part 1

Transcriber Note

Text Emphasis denoted as _Italics_ and =Bold=. Magnificatons as 30/1. The » is used in Swedish for quotes and dittos. Numeric abbreviations N:o, 1:o, 2:o, etc. for Number, primo, secundo, etc. Commas are used here as decimal separators (ex., 0,06 = 0.06.

KONGL. SVENSKA VETENSKAPS-AKADEMIENS HANDLINGAR. Bandet 34 N:o 8.

RESEARCHES

ON

THE VISUAL ORGANS

OF

THE TRILOBITES

BY

G. LINDSTRÖM.

WITH SIX PLATS

COMMUNICATED TO THE ROYAL SWEDISH ACADEMY OF SCIENCES

SEPTEMBER 12TH 1900.

STOCKHOLM

KUNGL. BOKTRYCKERIET. P. A. NORSTEDT & SÖNER

1901.

CONTENTS.

Introduction Page 6. Blind Trilobites » 9. The eyes of the trilobites » 26. On the maculæ of the hypostoma » 35. Conclusions » 71. Explanation of the plates » 75.

At the outset of this memoir it must be said, that it is in fact a joint work by the first discoverer of the remarkable hypostomic structure to be described, Herr G. LILJEVALL, and by myself. A considerable portion of this work belongs to him as he with great discernment searched for a large amount of the material, prepared it as to be fit for the many microscopic sections which he executed, and at last with his well known artistic skill drew the figures. My share, again, in the common work consists in the supervision of the figures, in the arrangement of the observations and details in a coherent form, in which suggestions and remarks of Herr LILJEVALL have been of great use, further to write the descriptions and to make all the necessary researches in literature, so whatever may be faulty in these respects must be laid to my charge.

September 12th 1900.

G. LINDSTRÖM.

INTRODUCTION.

It is well known amongst palæontologists that the hypostoma of the trilobites has been the particular object for research and description by a few authors, as BARRANDE, NOVÁK and BRÖGGER and some passing notes concerning it are given in the descriptive works of other authors. In the following pages reference is often made to them and I may here give a list of the chief works and the abbreviations with which they are cited below in this paper.

ANGELIN Palæontologia Scandinavica = ANG. BARRANDE Système silurien de Bohéme = BARR. BRÖGGER Die silurischen Etagen 2 & 3 im Christianiagebiet = BR. 1. ---- Die Ausbildung des Hypostoms bei einigen Asaphiden BR. II. HALL, JAMES, Palæontology of N. York, vol. I, II, VII = H. HOLM Trilobiter ur Dalarnes graptolitskiffer = HM I. ---- Ostbaltische Trilobiten, Illæniden, = HM II. NOVÁK, Studien an Hypostomen Böhmischer Trilobiten. Three different papers between 1884-1886. = N. II, N. III, N. IV. SALTER Monograph British Trilobites. SCHMIDT Ostbaltische Trilobiten I, II, IV. = SCHM. WOODWARD, HENRY, Carboniferous Trilobites. Pal. Soc. 1883-84.

* * * * *

Before proceeding further I shall give a general representation of the shape of the hypostoma. This enigmatic part of the trilobite skeleton resembles as to its outline a heraldic shield being broad at its anterior margin, gradually tapering towards the posterior extremity, which generally is acuminated or, as in the Asaphidæ, bifid. In a few, as Deiphon, Remopleurides, it is nearly square. Its exterior surface may consist of a single field or he divided transversally into two or three fields, according to the presence of one or two shallow grooves. These are transverse, curved forwards near the lateral margins and may be designated as the anterior groove and the posterior groove. Beside these grooves there are in some instances others, one on each of the lateral margins, parallel with these. On both sides of the anterior margin a protuberance, sometimes of considerable length, stands out. They are bent backwards towards the interior side of the hypostoma and in some genera, as Dysplanus, assume the shape of long, acuminated, halfways hollow horns. These protuberances were called »_ailes_» by BARRANDE and it is best to retain this term, in english »wings», in spite of its being not quite expressive. A smaller posterior pair lies nearer to the posterior margin and these wings are so much bent backwards, that they are seldom visible from the outside. They are prolongations from the »duplicature» or the narrow fold of the posterior margins round the interior side of the hypostoma.

For the rest the whole exterior surface is covered with terrace lines running concentrically and varying according to the different genera and even species. Many other genera, again, as Acidaspis, Calymmene, Homalonotus, Dalmanites etc., are devoid of them and instead granulated or smooth.

In the plurality of species there are two tiny patches or maculæ, sometimes elevated above the surrounding surface like tubercles and so they have also been called by some authors. But I have preferred to use the name »macula» for them, as the plurality does not form tubercles. They are generally entirely smooth and glossy and situated next to the anterior groove, either above it or in it, at a regular distance from each other and the lateral margins. They may form a sunk spot or, as commonly, an ovoid or elliptic area surrounded by a linear elevated border. Thus amongst the Asaphidæ. In others, again, (Bronteus, Illænus etc.) they form a moderately elevated tubercle. The direction in which they are oriented in relation to the longitudinal axis of the hypostoma is quite as variable. It is to the closer consideration of the nature of these macula that this memoir will be chiefly devoted. They have been delineated several times, but very seldom has any accurate attention been paid to them by previous authors. It is highly doubtful if BARRANDE ever alluded to them, when he in his »Système Silurien de Bohéme» vol. I, p. 156, in describing the hypostoma, says: »Cette plaque bombée porte souvent des saillies et des empreintes creuses, dans le voisinage de la bouche. Leurs formes varient suivant les espèces mais en conservant toujours les caractères génériques. Nous les considérons comme les points d'attaches des muscles et des màchoires.» In the numerous specific descriptions in his grand work there is never any mention made of the macula and in contemplating the figures and reading the explanations of these, we shall find that he meant something more comprehensive with his statement, to wit, the entire groove where these tubercular macula are situated.

In NOVÁK'S third paper, p. 4, we see the figures of two Phillipsiæ with tubercles on the hypostoma and he says only that in the anterior groove there are two such symmetrically placed ... glandular intumescences.[1]

[Footnote 1: »In der Mittelfurche sitzen 2 kleine symmetrisch gelegene, nicht immer dentliche drüsenartige An schwellungen.»]

BRÖGGER again in his paper on the »Ausbildung des Hypostoms» devotes a page (p. 19) to explain the nature of these maculæ and regards them as the exterior marks of two muscular impressions or points of attachment on the inside for muscles which have attached the hypostoma to the walls of the glabella or the head. The real nature of these so variable tubercular maculæ is, however, a quite different one, as we hope to be able to demonstrate in describing all their varying shapes in several genera.

The interior surface of the hypostoma shows all the protuberances of the exterior surface (tubercular maculæ, spines and granulations) as shallow pits, the exterior grooves being on the contrary elevated ridges. For the rest the interior surface is smooth and its margins are posteriorly covered with a more or less broad duplicature, from the lateral margins of which the posterior wings project.

The manner in which LILJEVALL became aware of the interesting structural features of which I am going to give an account, is as follows. We were about to describe and delineate some new or not sufficiently known Upper Silurian trilobites from the island of Gotland. In order to draw a specimen of Bronteus polyactin ANGELIN, which is rarely found in the Wenlock and Ludlow strata of Gotland, LILJEVALL was preparing and cleaning its hypostoma. The shape of this hypostoma, as represented in pl. II fig. 22, is clypeiform, tapering towards the posterior margin, which is regularly tongue shaped. The anterior margin is faintly curved and at both sides elongated in a broad, pointed wing. Its exterior surface is divided in three transverse fields, defined through two shallow grooves, the anterior and largest field occupying more than the moiety, the median one is crescent-shaped and narrow, and the posterior one is nearly of the same size and confluent with the lateral borders of the hypostoma, which near the median transverse axis of the latter project in a blunt angle on each side. The surface, for the rest smooth, is covered by concentric, irregular terrace lines. On the inferior edge of the upper groove two small tubercles are situated, one on each side closer to the lateral margins than to the central axis. The size of their longitudinal axis amounts to 0,96 mm. in the shortest and to 1,07 mm. in the longest specimens. This axis is parallel with the groove and consequently oblique to the longitudinal axis of the hypostoma. They are oblong or ellipsoid, their inferior apices bluntly pointed or rounded, varying a little as seen in the three specimens figured, (Pl. II figs. 23, 24, 25). For about two thirds their surface is perfectly smooth or rather glossy, and the lower third is covered with a compact accumulation of small granules. The extension of this peculiar accumulation varies, being somewhat larger in some specimens, and the limit which runs oblique towards the glossy part is more or less curved. Such a granule taken single is of the extremest minuteness, measuring 0,055 millimeter at the highest. Their outline is perfectly circular and semiglobose.

When LILJEVALL saw this group of granules he was at once struck with the perfect, outward similarity between them and the facets of the compound eyes on the upper surface of the head of this same species, and I fully concurred in the same view. It now became of importance to ascertain whether the intimate structure also corresponded in both, and microscopic sections were accordingly prepared. Although not so clear as in other species of this genus a vertical section of a cephalic eye (Pl. II fig. 21) shows a median row of black spots which compared with other sections (Pl. II fig. 9) must be the centres of the badly preserved lenses. This is confirmed by the horizontal section (Pl. II fig. 20) where the well known image of the polyedric facets appeared. In the same manner the granulated spot of the hypostoma shows in a vertical section even more clearly (pl. II fig. 27) the elongate, coherent lenses with a black centre and the polyedric form of the lenses in a horizontal section (Pl. II fig. 26). The assumption of a pair of small adventive eyes on the exterior side of the hypostoma became thus based on the clear evidence of the perfect structural agreement between them and the eyes of the head. But it was necessary to strengthen this evidence through extended researches on the hypostoma of as many trilobite genera as possible and also to study the intimate structure of the cephalic eyes in as many genera as could be accessible.

We shall now give first a general account of the structure of the eyes in the trilobites as far as we have been able to study them, and then proceed to describe the maculæ of the various genera, which we have observed.

With regard to the presence or non existence of visual organs, we must remark that a considerably greater number of genera than those which unanimously have been regarded as blind, also are devoid of eyes though they still by many authors are ranked amongst the oculate ones and we then had better first to make a review of them and thus to eliminate them from the number of the oculate species.

=Blind Trilobites.=

In the chaos of generic forms and in the great disagreement which prevails as to the systematizing of the trilobites of the Cambrian time, there is a thorough revision of them highly needed by a person having access not only to the literature, but also to the original specimens. It is almost impossible in the present state of things to tell with any degree of certainty how many well established genera had been living during that period. Hence the difficulty of fixing the systematic names of many specimens the visual organs of which are to be described.

My researches on the visual organs of the Cambrian trilobites are founded on the Angelinian Collection in the Swedish State Museum together with collections of foreign species, but also largely on the waste European and American literature, though we have to deplore the often occurring inexactitude of the figures, especially in the older works, and constructed or schematized figures in some of the newer ones, which give a quite false notion of the structure. There is no lack of figures to show how it ought to be, according to preconceived notions and, on the other hand, a great scarcity of representations, to show how it really is. In spite of all this there is a sufficiently great number of well established facts to demonstrate the organization of the Cambrian genera.

The trilobites of this division may be called blind only in so far as they have no eyes on the upper surface of the head, but they may have been provided with visual organs, though more imperfect, on the hypostoma as really seems to have been the case with some of them.

According to the structure of the head shield the blind trilobites may be subdivided into the following well characterized groups. These are:

I. Without facial ridge: 1) The Archæan Trilobites.

II. With facial ridge: 1) The Olenellidæ. 2) The Olenidæ and related.

=I. Blind trilobites without facial ridge (= »eye lobe»).=

=Group 1. The Archæan Trilobites.=

In these the head shield is in one piece without any facial suture and facial ridge, and without the least trace of anything that might be called a visual organ and they must consequently be considered as totally blind. In contradistinction to the following groups, excepting the oldest Olenellidæ, the head consists of only three parts, 1) glabella, and 2-3) the two fixed cheeks. These genera range from the oldest zone in which hitherto trilobites have been found, that of Olenellus (Holmia) Kjerulfi, to the zone of Paradoxides Forchhammeri, and some, as Agnostus, even continue as high in the Lower Silurian series as in the Brachiopod schists. Beside Agnostus the other genera are Conocoryphe (seven species in the Swedish Cambrium), Toxotis, Ctenocephalus (?), Elyx, Aneuacanthus, Conophrys and Microdiscus.

»Harpides» breviceps ANG., also belongs here. Anopocare of ANGELIN should also be regarded as one of this group. But it cannot be retained any longer because it is founded on two other, well known forms, being, according to LINNARSSON, Peltura scarabæoides (pl. 27 fig. 1, a in Pal. Scand.) and young specimens of Sphærophthalmus alatus (ibid. figs. 1 & 2).

It is remarkable that some of the Conocoryphidæ have an imperfect facial ridge, to be compared with the commenced one in Sao BARR. I (pl. 7 fig. 9). So the American Con. trilineata and reticulate. WALCOTT U. S. Geol. Survey 10th Rep. pt. I, pl. XCV f. 5 & 6. It is, as it were, arrested in the development and these adult trilobites had stopped, where the larva of Sao was proceeding in its second stage. They are the forerunners of the blind trilobites with facial suture, belonging to the third group. It needs scarcely be mentioned that the genera now enumerated have hardly anything in common, beside the general character of the head, and that real affinity exists only between Agnostus and Microdiscus, and probably also between Conocoryphe, Ctenocephalus and Elyx.

BEECHER[2] asserts that there is a suture in Agnostus, but in vain we have searched for it in numerous well preserved specimens and Dr HOLM also denies its presence. Nor are there any signs of closed up sutures, which also could not possibly be expected in so early a stage of evolution. It may then be taken as well settled that a fundamental character in these the oldest[3] of all known trilobites is the total want of a facial suture and a compactness of the whole head shield which later is broken up in several parts through the disjunction of the free cheeks. In the Lower Silurian formation there are a few genera sharing in the same structure of the head shield, though by no means else related. Such are Dindymene, Areia, Carmon and in the U. Silurian Cromus. The two species forming BARRANDE'S genus Dindymene are so dissimilar that Dind. Friderici Augusti had better to be removed to a new genus and the first described one to be retained as type of the genus Dindymene. The same is the case with Carmon, where the type species C. mutilus is blind and without free cheeks while the other species, known only by its fixed cheeks and glabella is one of the Olenidæ.

[Footnote 2: Nat. Classification of the Trilobites, p. 183.]

[Footnote 3: Oldest in that sense that they are the descendants of an archaic precambrian stock, the chief characteristics of which they have retained in the main unchanged and persisting long ages after the close of the Cambrian times, some, as Agnostus, continuing high up in the Lower Silurian.]

=II. Blind trilobites with facial ridge.=

This large division embraces the second and third groups or, with a few exceptions, all the rest of the Cambrian trilobites on account of a feature in the cephalic sculpture common to them all, though widely different as to its first origin in both. What forms the prominent and common characteristic of these two groups is the presence of the _facial ridge_, which emanates from the basis or the front of the foremost segment of the glabella and in a great variety of different shapes continues backwards near to the posterior border of the head. It has received several names as _eye-line, palpebral lobe, ocular ridge, eye-lobe, ocular fillet_ (MATTHEW). In German it is named _Augen-leiste_, in French _filet_ (BARRANDE) and in Swedish _ögonlist_.[4] Some authors make a difference between the more narrow part, calling it eye line, and the thicker posterior node, which they name the palpebral lobe proper.

[Footnote 4: That name is the most current amongst the swedish authors, together with »_palpebrallob_»; HOLM says _ögonlob_ and _frontallob_.]

As this peculiar ridge exists before any facial suture has made its appearance and separated the head shield in five parts, viz. the median glabellar part, the two fixed cheeks and the two free cheeks, and as it occurs in genera which never possessed any facial suture, and where no eye ever was formed, it is not adequate to call it an ocular ridge etc. the more so, as it, at least during a long series of genera succeeding each other, has had no connection whatever with any eye. I therefore propose to call it _facial ridge_ (in swedish _faciallist_). It occurs on the head of almost all Cambrian trilobites, excepting the archaic ones, and it is retained in the later Cambrian Peltura, Sphærophthalmus etc., which have real, compound eyes, as well as in a few Lower Silurian genera as Triarthrus, Pliomera,[5] Euloma, in the Upper Silurian Arethusina and Acidaspis and most persisting in Harpes, ranging from the Lower Silurian Lower Red Orthoceratite Limestone into the middle Devonian beds.

[Footnote 5: Pl. Törnquisti HOLM.]

It presents itself in the most variable shapes, and as it in fact can be followed through its development in the oldest known species, it is suitable to begin its description together with the characteristics peculiar to the oldest or second group in this large subdivision, that of the _Olenellidæ_.

=Group 1. The Olenellidæ.=

In this group we have two families of different age, the older, less developed the Olenellidæ proper, and the younger the Paradoxidæ. The former consists of the genera Olenellus, Holmia, Mesonacis and Schmidtia, and we shall attend to them first. They have no facial suture[6] and consequently a tripartite head shield like the Archaic ones, no eyes, but there is that strongly developed and most characteristic facial ridge. As seen in Olenellus Thompsoni[7] the crescent like ridge starts as a direct outflow from the base of the first segment of the glabella, and is in direct continuation with it as an integral part. It is roundbacked, regularly curved and at its starting point as broad as one of the posterior glabellar segments. It is regularly semilunate, tapers posteriorly and ends near the occipital segment. From the narrow second segment of the glabella, inside the just described larger ridge, a smaller ridge emanates, broader and flatter than the former, slightly curved, and ends between the third and fourth glabellar segment. Already in Holmia Kjerulfi the ridge is modified. There it is only one ridge, the anterior one, nearly as doubled through a shallow groove running along its back. The second one has dwindled away so as to be seen only as a narrow stripe near the occipital segment and ending outside this in a point. The anterior ridge, represented by HOLM as consisting of two nearly independent parts, is indeed in one piece, though its dorsal groove sometimes is deep, and it is with its total breadth joint to the first glabellar segment. Along its outer edge, where it lies close to the cheek, a narrow slit runs, and I suppose that it is to be considered as the first indication of the forming of a facial suture, which however does not reach longer than the ridge. As in Paradoxides it is so tight to the cheek, that there has been no place for an eye. It is no accidental break, its edges being too regular and unbroken in all specimens. This ridge is by far much shorter than in Olenellus and terminates opposite the third segment.

[Footnote 6: BEECHER Natural Classification of the Trilobites, p. 191, pretends that in Olenellus and Holmia real sutures »in a condition of symphysis» occur. He seems to deny the facial sutures and to accept as »real sutures» the »internal sutures» described by HOLM in Olenellus Kjerulfi. It is highly doubtful if these interior elevated lines are to be regarded as sutures. They are indeed no sutures, but in reality elevated linear ridges, inclosing, as it were, narrow canals. The real sutures known, the facial sutures, never form elevated lines, be it on the outside of the head or on the inside. Probably these lines are derived from some now unknown interior organization and it may be fit to remind of the somewhat similar though more numerous linear canals of the branchiæ on the interior surface of the great head shield of Apus. (ZADDACH De Apodis cancriformis Anatome ... pl. II fig. 1) or what HUXLEY (Anatomy Invertebr. Animals p. 281) calls the convoluted »shell gland» in the carapace. A quite different structure is what I suppose to be the incipient facial suture in Holmia.]

[Footnote 7: WALCOTT in 10th Ann. Rept. U. S. Geol. Survey, pl. 82 f. 2.]

1. Hypothetical figure, based on fig. 2. represents a stage preceding the next. There are three pair of pleura and three segments of the rhachis.

2. Copy of WALCOTT'S fig. 5 in »Bullet. 1886», pl. XVII. Three pleura at right, two at left, the larger formed through fusion of the two hindmost in fig. 1.