1852.] M'COY SILURIAN FISH AND CRUSTACEANS. 13 



Silurian Crustacean which I have figured in the first Fasciculus of 

 the 'Cambridge Palaeozoic Fossils,' PL 1. E. fig. 7, under the name 

 Pterygotus (Leptocheles) leptodactylus, it occurred to him that exa- 

 mination might prove some of the other remains described by Prof. 

 Agassiz as fishes in the 'Silurian System' to belong to the same 

 class, and he accordingly sent me all the specimens, now accessible, 

 figured on that plate, to examine and report upon. The specimens 

 sent to me only belonged to the Thelodus parvidens and Onchus 

 tenuistriatics, together with a fragment called on the plate " an 

 Ichthyodorulite " (figs. 63 & 64), but I venture a few observations 

 on all. 



Figs. 1, 2, & 3, supposed to be shagreen of Sphagodus, I cannot 

 determine without seeing the specimens. 



Figs. 4 & 5 ; although these scale-shaped markings are stated to 

 belong to di.Jish called Pterygotus problematicus, still M. Agassiz, in 

 his volume on the Fishes of the Old Red Sandstone, very properly 

 removes this genus from the class of Fishes and places it in its true 

 class, Crustacea. I have suggested, in the work on the Cambridge 

 Fossils, that Pterygotus belonged, not to the Macrurous Crustacea, 

 but to the group Pcecilopoda, allied to the recent Limulus or King-crab, 

 and recent discoveries published in this Journal confirm this opinion* : 

 the supposed fish-tooth (fig. 6) called Sphagodus pristodontus would 

 in this case, almost certainly, be not a fish (to no known tooth of 

 which has it any accurate analogy), but the masticating, serrated 

 edge of the basal joint of one of the feet surrounding the mouth of 

 the same Pterygotus, the sculpturing of whose carapace is represented 

 by figs. 4&5. 



The genus Pterygotus is divisible into two subgenera : 1 . Ptery- 

 gotus proper, in which the didactyle claws are very thick and armed 

 with powerful teeth ; 2. Leptocheles (M'Coy), in which the pincers 

 are very slender and unarmed. As before mentioned, figs. 9, 10, «& 11, 

 representing the so-called Onchus Murchisoni, Ag., are almost iden- 

 tical in form, size, sculpturing, and all other characters (as far as 

 they are represented in these drawings), with the distinctly didactyle 

 pincers which I have figured (Brit. Pal. Foss. pi. E. fig. 7) from 

 Leintwardine, under the name Lept. leptodactylus, in which the two 

 fingers occur, in situ, removing all doubt as to its true nature. If 

 this approximation prove correct, the fossil should in future be called 

 Leptocheles Murchisoni (Ag. sp.) ; and I might add, that the num- 

 ber and relative position of the fragments on the stone figured, in- 

 stead of being singular, as supposed, would thus be nearly natural. 

 I might further remark, that the drawing shows no attenuation at 

 the base of these supposed fish remains, nor any of the other distin- 

 guishing characters of Ichthyodorulites. As this specimen unfortu- 

 nately cannot now be referred to, great importance attaches to the 

 fragment (fig. 64) called simply " an Ichthyodorulite " on the plate, 

 the right-hand extremity of which so exactly coincides vidth the sup- 



* See Mr. Salter's observations on Pterygotus, Quart. Journ. Geol. Soc. vol. viii. 

 p. 387. 



