Part 9

The position of the hypostomic eyes on the ventral surface of the trilobites is not quite so abnormous, nor so isolated a feature amongst the crustaceans as might at first be supposed. Amongst the recent crustacea there is probably none, which as to the conformation of its labrum or hypostoma resembles the trilobites so closely as the species of the genus Apus, however different they may else be in all other respects. We have studied chiefly the hypostoma of Apus cancriformis. In its shape as seen from the outside and as to its outlines it reminds strongly of the trilobite hypostoma. It is nearly square (pl. VI figs. 46, 47) with rounded corners. The anterior margin is arched and in the middle it has a broad tonguelike prolongation. On the sides of the anterior margin there are two short, sharply pointed wings. The lateral margins are thick, almost as doubled, and bent forward so that a groove is formed alongside them and the large, evenly vaulted median surface of the hypostoma. These grooves are continued in a short groove parallel with the posterior margin. From this margin a narrow oblique surface is slanting backwards, as it were, doubling that margin. On its surface there are two small, resplendent, white or clear spots, almost of the shape of a crescent, though more irregular when seen in higher enlargement (pl. VI fig. 49). They are translucent as to be visible on the interior side of the hypostoma. In Apus glacialis there are also similar spots in the same position, but they form short narrow, straight stripes, parallel with the border of the posterior margin. I have no opinion as to the nature or function of these white spots. I do not think that they at all are homologous with the macula; of the trilobites. On the other hand we see a little higher up on the exterior side of the hypostoma two faintly elevated elliptic tubercles which as to their position and shape highly remind of the maculæ of the trilobites. They are hollow on the inside and we have not been able to find any peculiar structure in them. That they have any visual function is precluded through the circumstance that the whole exterior surface is densely covered with microscopic bristles (pl. VI fig. 49) which are especially remarkable along the posterior margin.

In the class of the Cirrhipedia a sufficient number of facts is known to ascertain us of the presence of eye spots on the hypostomas of the larva. The researches of SPENCE BATE[43] show in the larva of Balanus balanoides, on the hypostoma or labrum, which he calls proboscis, an eye, a little from the posterior margin of the labrum. It seems during the various moults of the animal to have changed its place, being in the two first moults situated near the anterior margin of the hypostoma. In Clitia Strömi it lies at the anterior part of the labrum (figs. 9, 10). SPENCE BATE does not accept the opinion of their nature as eyes, but the following authors as DARWIN and CLAPARÈDE agree in considering these spots as eyes. DARWIN[44] describes two such eyes in Lepas australis and, if I catch his meaning rightly, one on each side of the labrum. CLAPARÈDE has given a figure of the larva of Lepas anatifera[45] in the hypostoma of which (by him called »Rüssell», proboscis) a single eye is seen, having at the sides two round problematic organs, that he at first considered as auditory cells, but later as eyes. HESSE[46] has also in a species of Scalpellum seen several coloured spots on the inferior surface of the carapace, below the antennæ and he expressly says that they are eyes and a special pair of supplemental eyes that are becoming functional after the first moult.

[Footnote 43: On the development of the Cirripedia, Ann. Mag. Nat. list. 1851, p. 324, pl. VI figs. 1, 2, 3, pl. VII fig. 10.]

[Footnote 44: Monogr. of Cirripedia I p. 17.]

[Footnote 45: Beobachtungen ... über wirbellose Thiere, 1863, pl. XVII fig. 22.]

[Footnote 46: Mémoire sur les métamorphoses que subissent les Anatifes appelés Scalpels obliques. Annales des Sciences nat. 1859. Tome XI p. 163.]

These sternal eyes in the larva are previous to the eyes in the dorsal side of the later stages and whenever such are discovered in other groups of adult animals as the trilobites they must of course be considered as remnants from the larval stage. As a rule, in several tribes of the crustacea it has been observed that the eyes originate on the ventral or sternal side of the body and in the advancement of growth move to the superior side. So according to PACKARD and others the embryo of Limulus has originally the median eyes on the ventral side. In the same way the cephalic eyes of Dalmanites move, as BARRANDE has shown, from the ventral side to their permanent place on the superior side of the head shield. But while in the Cirrhipedia the larval or hypostomic eyes are lost in the adult they remain on the hypostoma of a great number of the adult trilobites and have probably left their mark behind in A pus. No wonder then that we find these maculæ developed as precursors of the hypostomic eyes in a number of the oldest and in other respects blind trilobites as the Olenellidæ and Paradoxidæ to which they may have been of some use as a sort of visual organs, though probably very imperfect.

We find the maculæ of the trilobites present from the oldest Cambrian times and we find also in them a progressive evolution, in some to a high degree, lenses and facets, perfectly identical with those of the eyes on the head shield, converting them into true eyes. It was in the Lower Silurian times, in the Lower grey Orthoceratite limestone of Sweden, that Illænus Esmarki appeared provided with actual prismatic lenses on its maculæ and probably also Lichas. Still higher at the top of the Lower Silurians, in the Leptæna limestone of Dalecarlia the genus Bronteus has appeared with those highly developed hypostomic eyes.

But there are, no doubt, still more facts to adduce for filling up extant lacunæ in the knowledge of these matters. The material now at hand has, however, not been sufficient for doing this. At present I must leave this interesting field of investigation and be contented if I have succeeded in showing where rich harvests await every one who will make a search for them.

Explanation of the plates.

All specimens figured belong to the Swedish State Museum, unless otherwise stated.

Pl. I.

=Acidaspis crenata= EMMR. p. 85.

Fig. 1 & 2. Vertical sections of the eye 60/1. » 3. The outside of the hypostoma 6/1. » 4. The anterior margin of the hypostoma, showing one of the clubshaped spots in transmitted light 20/1. » 5. The granulations of the hypostoma in transmitted light 80/1. » 6. Vertical section of the hypostoma with granules 60/1. Specimens from Djupvik, Eksta.

=Agnostus glandiformis= ANGELIN p. 35.

Fig. 7. Vertical section of a part of a rolled up specimen 8/1. Specimen signed CQ. Andrarum, Scania.

=Asaphus expansus= L. p. 39.

Fig. 8. Horizontal section of the eye near the upper margin. _a._ The surrounding spongious zone, _b._ The surface of the eye with lenses 60/1. Spec. CG. » 9. Horizontal section from the centrum of the eye 60/1. Spec. CF. » 10. Horizontal section near the inferior margin, _a_ the eye, _b_ marginal zone, _c_ a portion of the cheek 60/1. Specim. CF. » 11. Vertical section near the inferior margin 60/1, same specimen as in fig. 10. » 12. Eye fractured vertically showing regular prisms in the eye (_a_) and irregular ones in the marginal zone (_b_) 30/1. » 13. Part of the hypostoma with the macula 12/1. » 14. The right macula from the inside 12/1. » 15. A vertical section across the hypostoma, _aa_ the maculæ 8/1. » 16. A similar one of the macula at left in the preceding figure 60/1. » 17. The right macula from the outside in transmitted light 60/1. Sp. BQ.

=Asaphus fallax= DALM.? p. 41.

Fig. 18. Horizontal section of the eye 60/1. Spec. CD. » 19. Vertical section of the same passing obliquely through two rows of prisms 60/1. » 20. Fragment of the same eye with entire prisms 30/1. » 21. The hypostoma seen from the inside 3/1 from Sandvik, Öland. » 22. The left macula 12/1. Spec. BO. From Vestanå, Östergötland.

=Asaphus raniceps= DALM. p. 40.

Fig. 23. The left macula 12/1. Spec. BV. » 24. Horizontal section of right macula 60/1. Spec. BY. » 25. Vertical section of a macula 60/1. » 26. A similar section of a macula (_a_) in the longitudinal axis of the hypostoma 12/1. Exterior surface at right. All from Östergötland.

=Asaphus= sp. p. 41.

Fig. 27. A portion of the eye near the superior margin 30/1. » 28. Horizontal section of the eye 60/1. » 29. Horizontal section near the inferior margin 60/1. » 30. Vertical section near the superior margin 60/1. All figures from specimen CE. From Gotska Sandön.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. I.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

Pl. II.

=Asaphus= (Isotelus) =gigas= J. HALL p. 42.

Fig. 1. The right macula surrounded by terrace lines 3/1. » 2. Horizontal section of a macula with a part of the test 60/1. » 3. Vertical section of the same, the surface of the outside upwards 60/1. All specimens from Cincinnati, Ohio, Hudson River group. Spec. BU.

=Bronteus irradians= LINDSTRÖM. p. 43.

Fig. 4. The hypostoma 4/1. » 5. The left macula 12/1. Specimen from Gotland, Löjsta.

=Bronteus laticauda= ANG. p. 44.

Fig. 6. Horizontal section of the cephalic eye 20/1. » 7. The same highly magnified 250/1. » 8 & 9. Vertical sections of the cephalic eyes 20/1. In fig. 8 with integument, in fig. 9 decorticated. » 10. Hypostoma. Specim. N. 2/1. » 11. The right macula, from the outside. Spec. O. 12/1. » 12 & 13. Left maculæ, casts of the interior surface 12/1. Specimens from Dalecarlia.

_Bronteus platyactin_ ANG. p. 43.

Fig. 14. Vertical section of cephalic eye, _a_ the eye. _b_, cheek 20/1. » 15. Hypostoma 2/1. » 16. The same from the interior surface. Sockets of the maculæ sunk in the shell 2/1. » 17. The right macula 12/1. » 18. Portion of the eye on the macula in transmitted light 20/1. » 19. The same more magnified 60/1. Specimens from the stratum _b_. Wisby, Norderstrand.

=Bronteus polyactin= ANG. p. 8.

Fig. 20. Horizontal section of cephalic eye 20/1. » 21. Vertical section of the same 60/1. » 22. Hypostoma 4/1. » 23-25. Samples of right maculæ 20/1. » 26. Horizontal section of macula, the portion bearing the lenses 20/1. » 27. Vertical section of the same 60/1, _a_ the lenses. Specimen from Stora Carlsö. » 28. Hypostoma 4/1. » 29. The right macula 12/1. » 30. A portion of the lenses on the macula in reflected light 30/1. The lenses are only faintly convex and encased in a white frame. The diameter of the largest lenses 0,06 millim. Specimen from Lansa, Fårö.

=Bumastus barriensis= MURCH. p. 46.

Fig. 31. Hypostoma 4/1. » 32. The right macula 12/1. Specim. CU from Wisby.

=Bumastus sulcatus= LINDST. p. 46.

Fig. 33. Horizontal Section of a cephalic eye 20/1. » 34. The same magnified 60/1. » 35. A similar, near the inferior margin and close to the surface, _a_ the eye, _b_ cheek 60/1. Compare fig. 38. » 36. Vertical section of the cephalic eye near the suture 20/1. » 37. The same, more magnified 60/1. » 38. A similar, near the inferior margin, _a_ eye, _b_ cheek 60/1. » 39. Cast of the interior surface of a hypostoma 2/1. » 40. The same seen from the side. Originals to figs. 33-38 from Samsugu, Othem, Gotland, figs. 39 & 40 from Stora Vede, Follingbo, Gotland.

=Bumastus= sp. p. 47.

Fig. 41. Horizontal section of cephalic eye, near the inferior margin, _a_ eye, _b_ cheek, 60/1. » 42. Vertical section of the same 60/1. » 43. A similar near the inferior margin, _a_ eye, _b_ cheek, 60/1. » 44. Hypostoma 4/1. » 45. Side view of the same. » 46. The right macula 12/1. Specimens from Korpklint near Wisby.

=Bumastus= sp. p. 46.

Fig. 47. Hypostoma 4/1. » 48. The right macula 12/1. Specimen from Klints, Othem, Gotland.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. II.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

Pl. III.

=Bumastus= n. sp. p. 47.

Fig. 1. Hypostoma 4/1. Specimen AY. From Wisby, Norderstrand. » 2. Its left macula 12/1.

=Bumastus= n. sp. (different from the preceding) p. 47.

Fig. 3. Hypostoma 4/1. Spec. AT from Wisby, Norderstrand. » 4. Vertical section of the macula _a_ in its longest axis. » 5. A portion of the cephalic eye, the facets very low.

=Calymmene intermedia= LINDSTR. p. 48.

Fig. 6. Hypostoma 4/1. Spec. BG from Gotland. » 7. The left macula 12/1.

=Calymmene tuberculata= BRÜNNICH p. 48

Fig. 8. The inside of the hypostoma 3/1. Spec. BA from Gotland. » 9. Horizontal section near the surface, seen from the inside 20/1.

=Chasmops macroura= (?) p. 49.

Fig. 10. The left macula, cast of the inside 12/1. Rostock.

=Chirurus glaber= ANG. p. 50.

Fig. 11. A portion of the surface of the cephalic eye 12/1. From Dalecarlia.

=Chirurus ornatus= DALM. p. 50.

Fig. 12. Horizontal section of a Cephalic eye near the surface 60/1. » 13. Vertical section of the same, 20/1, at the inferior margin. A line added to show the supposed size of the fragmentary lenses. » 14. The left macula of the hypostoma 12/1. From Östergötland, Westanå.

=Chirurus spinulosus= NIECZKOWSKI p. 51.

Fig. 15. The right macula of the hypostoma from the inside 12/1. Belongs to Dr. G. HOLM, from Esthonia, Kuckers C^2.

=Chirurus= sp. p. 51.

Fig. 16. The left hypostomic macula 12/1. Öland, Brunsby kanal.

=Cyrtometopus clavifrons= DALM. p. 49.

Fig. 17 & 18. Horizontal sections of cephalic eyes 60/1. » 19. Vertical sections of cephalic eyes and a part of the cheek 20/1. » 20. A highly magnified portion of the eye in vertical section 60/1. » 21. Hypostoma, 3/1 the anterior wings completed after another specimen. From Östergötland.

=Cyphaspis elegantula= ANGELIN p. 52.

Fig. 22 & 23. Horizontal and Vertical sections of cephalic eyes 60/1. Diameter of facets 0,03 mm. » 24 & 25. Hypostomas from the front side and in side view 8/1. Upper Silurian, Gotland.

=Ctenopyge pecten= SALTER p. 29.

Fig. 26. The head shield, the free cheeks added from other specimens 4/1. » 27. A free cheek, viewed without foreshortening, as in its natural position 4/1. Alumschists of Andrarum, Scania

=Ctenopyge= n. sp. (LINNARSSON, Geol. För. Förhandl. 1880, pl. VI, figs. 14, 15) p. 29.

Fig. 28. A free cheek 6/1. » 29. Surface of the eyes, near the suture 30/1. » 30. The same next the free cheek 30/1. Andrarum.

=Sphærophthalmus alatus= ANG. p. 29.

Fig. 31. The head shield; the free cheeks have been added 8/1. » 32. A free cheek without foreshortening 8/1. » 33 & 34. Horizontal and Vertical sections of cephalic eyes 100/1. The cornea nearly 0,01 mm. in thickness. Andrarum.

=Peltura scarabæoides= WAHLENB. p. 29.

Fig. 35. The head shield; the free cheeks added 2/1. » 36. A free cheek, not foreshortened 4/1. » 37. A cast of the inside of the cephalic eye 20/1. » 38. A portion of the cephalic eye from the inside 60/1. » 39. A portion of the same in transmitted light 100/1. » 40. Vertical section of an eye 100/1. The rock on the inside. » 41. A similar one passing through two rows of lenses, the one behind the other. The rock on the outside 100/1. » 42. A hypostoma 8/1. Andrarum.

=Dalmanites imbricatulus= ANGELIN p. 53.

Fig. 43. The cephalic eye near the inferior margin 20/1. » 44. Vertical section of the same and a portion of the free cheek 60/1. The lenses are almost contiguous and the interspace between them is formed by the common test of the body, which is perforated by longitudinal tubes.

=Dalmanites obtusus= LINDSTR. p. 53.

Fig. 45. Horizontal section of cephalic eye near the inferior margin 30/1. » 46. Vertical section, the interspaces being in this wider than in the preceding 60/1. From Gotland.

=Dalmanites vulgaris= SALTER P. 54.

Fig. 47. Part of the cephalic eye near the inferior margin 20/1. » 48. Horizontal section near the surface at the inferior margin 30/1. » 49. A tangential section, so that the inferior part is below the lenses where the peculiar radiating structure is seen in horizontal section 30/1. » 50. Vertical section of a small Specimen, fibrous appendices below the lenses 30/1. » 51. Hypostoma 2/1. » 52. The right macula 12/1. English specimens from Dudley, Shropshire.

=Dysplanus centrotus= DALMAN p. 55.

Fig. 53 & 54. Horizontal and vertical sections of an eye 60/1. » 55. Hypostoma 3/1. » 56. The anterior wing of the hypostoma seen in foreshortening 6/1. Specimen AX from Westanå, Östergötland.

=Dysplanus ladogensis= HOLM p. 55.

Fig. 57. Hypostoma 3/1. » 58. The same, seen from the side 3/1. Specimen AI from Östergötland.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. III.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

Pl. IV.

=Chirurus speciosus= HIS.

Fig. 1. The right macula, cast of the interior side 12/1. Specimen from Grumpevik, Vamlingbo, Gotland.

=Cybele bellatula= DALM. p. 52.

Fig. 2. The hypostoma 3/1. Vestanå, Östergötland.

=Dalmanites sclerops= DALMAN p. 54.

Fig. 3. The hypostoma 2/1.

=Encrinurus punctatus= WAHLENB. p. 56.

Fig. 4. Horizontal Section of an eye 69/1. Diameter of the prisms 0,04 mill., their length 0,07 millim. » 5. The hypostoma 3/1. » 6. The left macula. Reflected light 12/1. » 7. Part of the left macula in transmitted light 20/1. The black streaks are sectioned pores. Spec. BH. » 8. Vertical section of the right macula, a sagittal section 20/1. BH. » 9. A similar one _a_ along the shorter diameter 20/1. Specimens 5, 6 from Vikers Boge, 4, 9 Medebys, Hall, 7, 8 Svarfvarehuk. All from Gotland.

=Encrinurus lævis= ANG. p. 56.

Fig. 10. Horizontal section of the cephalic eye. » 11. Vertical section of the same. Both 60/1. Prisms 0,05 in breadth, 0,06 in length. Specimen from Wisby. » 12-17. Longitudinal profiles and anterior margins of the hypostomas of Encr. punctatus (figs. 12, 13), E. obtusus (figs. 14, 15) and E. lævis (figs. 16, 17).

=Harpes vittatus= BARR. p. 31.

Fig. 18. The two ocelli at right 8/1. » 19. Sagittal Section of two ocelli 60/1. Specimen from Lochkow E, e^2, Bohemia.

=Homalonotus Knighti= KÖNIG p. 57.

Fig. 20. The hypostoma 3/2. » 21. The right macula, from the interior side in transmitted light 20/1. From Scania.

=Illænus chiron= HOLM p. 58.

Fig. 22. Horizontal Section of an eye. » 23. Vertical section of the same, both 60/1. Spec. AG. » 24. Hypostoma 2/1. Spec. AF. » 25. The right macula seen in reflected light. The lenses translucent 12/1. Ex. AA. All from Persnäs, Öland.

=Illænus Esmarki= SCHLOTH. p. 511.

Fig. 26 & 27. Horizontal sections of the eyes in two different specimens 60/1. » 28. Vertical section of an eye 60/1. » 29. Likewise, _a_ the eye, _b_ a portion of the cheek 12/1. » 30. Hypostoma 2/1. » 31. The inside of the right macula 12/1. Spec. AR. » 32. The outside of the left macula in transmitted light 60/1. » 33. Sagittal section of the macula (_a_) 60/1. All specimens from Vestanå, Östergötland, excepting fig. 30 belonging to Dr. HOLM, from Norway, Bjerkaas.

=Illænus gigas= HOLM P. 59.

Fig. 34 & 35. Surface views of the prisms of the eyes, fig. 34 at the superior margin and 35 at the inferior one 12/1. » 36. Cast of the inside of a hypostoma 1/1. Same original as HOLM'S fig. 6 Tafl. I, Illænider. » 37. Cast of the inside of the left macula. Same specimen 12/1.

=Illænus Roemeri= VOLLBORTH (Ill. vivax HOLM) p. 59.

Fig. 38. Hypostoma 1/1. Spec. AO. » 39. The right macula; the lenses translucent in reflected light 12/1. Spec. AO. » 40, 41. Both maculæ of another, larger specimen 12/1. Sp. AP. From the Leptæna limestone of Dalecarlia.

=Illænus sphæricus= HOLM p. 59.

Fig. 42. The prisms of cephalic eye 12/1. } » 43. The right maculæ, transparent in } transmitted light 12/1. Sollerön, } Spec. AH. Dalecarlia. }

=Lichas= sp. p. 60.

Fig. 44. The hypostoma 1/1. » 45. Cast of the inside of the right macula 12/1. Spec. T. » 46. Cast in wax of the preceding 12/1. » 47. Cast of the inside of the hypostoma. The central portion with the maculæ 3/1. Spec. V. » 48. Inside of the right macula 12/1. Spec. Y. From a quarry north of the gallows near Wisby.

=Lichas latifrons= ANG. p. 60.

Fig. 49. The right macula, transmitted light, seen from the inside 60/1. From Kyrkberget, Wisby.

=Platymetopus planifrons= ANG. p. 67.

Fig. 50. The hypostoma 1/1. » 51. The left macula. Transparent lenses? 12/1. From the Leptæna limestone of Dalecarlia.

=Trochurus pusillus= ANG. p. 70.

Fig. 52. The hypostoma 5/1. » 53. The right macula in transmitted light. Seen from the inside 60/1. From Fröjel.

=Trochurus?= p. 70.

Fig. 54. The hypostoma 3/1. » 55. The right macula from the inside 12/1. From När Gotland.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. IV.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

Pl. V.

=Megalaspis attenuata= WAHLENB.