14 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 1, 



posed Onchus Murchisoni in diameter, scixlptui'iag, and number of 

 ridges, that it is exceedingly probable they are the same. Now this 

 specimen I have examined, and have no hesitation in stating deci- 

 sively that it is not an Ichthyodorulite, and that it is one of the 

 fingers of the claw of a Leptocheles ; for in the first place, the base, 

 instead of being abruptly attenuated like the dorsal ray of a fish, 

 is dilated, and both it and the longitudinally grooved portion are 

 merely the internal cast of a hollow, fragile, cretaceous crust, only 

 equalling the loioer dark outline of the drawing {or stout paper) in 

 thickness, as in the legs of ordinaiy Crustacea. These two characters 

 are decisive against the fossil being a fish-defence. — 'Fig. Q3, called 

 an " Onchus,'' is on the same stone mth the last specimen, and 

 from the analogy of the second pincers which I have figured as 

 above of Leptocheles leptodactijlus, I have no doubt it is the last or 

 moveable joint of a smaller pair of claws of the same Leptocheles 

 Murchisoni, for the analogy pointed out between the Pterygoti and 

 the Litnidi prepares us to expect several pairs of didactyle pincers of 

 different sizes and proportions in the one individual. 



I have mentioned that the typical Pterygotus had the claws aimed 

 with strong teeth, as in the common lobster ; this is well seen in 

 Agassiz' figure of the pincers of the Pterygotus Anglicus of the Old 

 Red Sandstone, and to the genus as thus restricted I have no doubt the 

 supposed fish teeth and jaws belong, figured under the numbers 14, 

 15, 16, 17, 18 to 32, and 60, 61, 62, under the names Plectrodus 

 mirabilis, P. pliopristis, and Sclerodus pustidiferus . Why separate 

 generic names should have been given to fragments so identical as 

 Plectrodus mirabilis (figs. 15 & 16) and Sclerodus pustidiferus 

 (fig. 62), I cannot divine. As one of these specific names expresses 

 a character distinguishing this Pterygotus from the large one of the 

 Scotch Old Red Sandstone, it shoidd have the preference, and these 

 fossils might stand in future lists as Pterygotus pustuliferus (Ag. sp.) 

 = Plectrodus mirabilis -\- P. pliopristis -\- Sclerodus pustidiferus, 

 Ag. Any reflecting comparative anatomist looking at fig. 14, will 

 agree, I think, in the opinion that no known fish-tooth, recent or 

 fossil, has the slightest structural analogy to warrant comparison 

 with it for a moment ; and on the other hand, the most casual ob- 

 server can trace identity with the tooth-like tuberculation on the 

 claws of the common lobster. 



Having disposed of the spurious fish-remains, I may state that the 

 Onchus tenuistriatus (figs. 12, 13, 57, 58, 59) is an undoubted Lch- 

 thyodorulite. Not only all the external characters indicate this, but on 

 submitting a transparent section of one of the fragments scattered 

 through the rock to a high magnifying power (with the kind aid of 

 Mr. Carter), I am able to state positively the existence of the Pur- 

 kinjian bodies and true microscopic structure of bone therein. This 

 example of a Silurian Fish is, therefore, perfectly correct ; and it only 

 remains to add, that all the specimens of the Downton Castle rock 

 which I have examined, impress me strongly with the conviction that 

 the last name on the list, the Thelodus parvidens, should be con- 

 sidered not as that of a fish-tooth, but of granules of the skin or 



