22 A. S. PACKARD, JR., ON THE ANATOMY 



all the remaining Trilobites, excepting of course the eyeless genera, Agnostus, 

 Dindymene, Ampyx and Dioride. The eyes of Phacops and Dalmanites are said by 

 Quendstedt and Barrande not to be compound eyes in the truest sense, but aggregated 

 eyes {Omli congregati). But judging by Barrande's figures of the eyes of Phacops 

 fecundus and P. modestus (Barrande, Vol. i, Suppl. Plate 13, figs. 12 and 22), and 

 our observations on the exterior of the eye of an undetermined species of Phacops, 

 kindly sent us by Mr. J. F. Whiteaves, Palaeontologist of the Canadian Geological 

 Survey, we do not see any essential difference between the form and arrangement of the 

 corneal lenses of Phacops and Asaphus, and are disposed to believe that the distinctions 

 pointed out by the above named authors are artificial. 



For my material I am mainly indebted to Mr. CD. Walcott, who has so satisfactorily 

 demonstrated the presence in Trilobites of jointed cephalothoracic appendages. On 

 applying to him for specimens, and informing him that I wished to have sections made 

 of the eyes of Trilobites to compare with those of Limulus, he very generously sent me 

 his own collection of sections of the eyes of Asaphus gigas and Bathyurus longistrinosus, 

 which he had prepared for his own study, also other eyes, and especially the shell or 

 carapace of a large Asaphus, from Trenton Falls, showing the eye and the projecting 

 points of the corneal lenses. Prof Samuel Calvin kindly sent me the eyes of an 

 unknown Trilobite from the Trenton limestone, one specimen showing the pits made in 

 the mud by the projecting ends of the corneal lenses, while to Mr. Whiteaves I am 

 indebted for eyes of Calymene. 



First turning our attention to the casts and natural sections ; that of the interior of the 

 carapace, including the molted cornea of Asaphus gigas, is noteworthy. When the 

 concave or interior surface of this specimen is placed under a magnifying power of fifty 

 diameters, the entire surface is seen to be rough with the ends of the minute solid conical 

 corneal lenses which project into the body-cavity. This is exactly comparable with the cast 

 shell of Limulus and its solid corneal lenses projecting into the body-cavity (plate 6, fig. 1). 

 Those of Asaphus only diflFer in being much smaller and more numerous, and perhaps 

 rather more blunt. Without much doubt the ends of the corneal lenses of Asaphus, as 

 in Limulus, were enveloped in the retina, the animal molting its carapace, the hypo- 

 dermis with the retina being retained by the Trilobite, while the corneal lenses were cast 

 with the shell. 



In the specimen of the unknown Trilobite from Iowa received from Prof. Calvin, the 

 corneal lenses, seen externally, are quite far apart, arranged in quincunx order ; the 

 lenses are round and decidedly convex on the external surface. In a natural section, 

 where the eye has been broken into two, the conical lenses are seen to extend through 

 the cornea as cup-shaped or conical bodies, and are quite distinct from the cornea itself. 

 In another broken eye of the same species, the cornea is partly preserved, and two of the 

 corneal lenses are seen to extend down into and partially fill two hollows or pits ; these 

 pits are evidently the impressions made in the fine sediment which tilled the interior 

 of the molted eye or cornea ! 



Thus in the Asa^jhus gigas noticed above, we have the entire inside of the cornea with 

 the cone-like lenses projecting from the concave interior; while in the last example we 

 have the impressions made by the cones in the Silurian mud which silted into the cornea 

 after the Trilobite had cast its shell. 



