Air. J. W. Davis on Pal?eospinax priscus, Egerton. 431 



dinal striations. The small tubercles mentioned by Sir P. 

 Egerton as besetting the lower portion of the posterior spine 

 do not appear to be present on my specimen ; but the covering 

 of shagreen presents a similar appearance to the one described 

 in the decades. The spine attached to the anterior dorsal is 

 I'l inch in length and "2 inch wide at the base ; it is 

 slightly curved backwards, gradually conti-acts in size, and 

 ends in a fine point. The second spine is similar in form but 

 larger; it is 1'3 inch in length. 



The pectoral fins are very large in proportion to the other 

 parts of the fish ; the anterior margin is at least 2'5 inches in 

 length. They are thickly covered with shagreen, the tubercles 

 clustering thickly and of large size along the anterior margin, 

 and becoming more thinly distributed and of gradually de- 

 creasing size towards the posterior portion of the fin. 



The ventral fins are also covered with tubercles in the same 

 manner; they are 1"1 inch in length. 



The dermal tubercles forming the shagreen are composed of 

 little plates, highly enamelled, for the most part rhomboidal in 

 form, in rare instances arranged like flat cubes in a tessellated 

 pavement, but more generally with the corner towards the tail 

 raised, as though the little cube were suspended diagonally at 

 two of the comers transversely to the longitudinal axis of the 

 body, and the anterior corner depressed, causing the one 

 towards the caudal portion of the fish to be raised. The raised 

 portion appears to be somewhat produced in some of the 

 tubercles ; but in no instance can I find evidence that any 

 thing approaching a sharp tooth-like structure occurred in any 

 of the tubercles, such as may be seen in many of the recent 

 Placoids. 



The figures (PI. XX. figs. 2, 3, 4) show the relative size of 

 the tubercles in different parts of the fish and also indicate a 

 slightly diflferent form. The minuteness of these objects may 

 be inferred from the fact that there are between 30,000 and 

 40,000 to cover one square inch of the body of the fish. 



The fossil Palceospinax presents an instance of a Lias fish 

 which bears so close a resemblance in general character, and 

 even to a large extent in minute detail, to the living AcanthiaSj 

 that we are almost justified in considering the latter as a de- 

 scendant of the former. The fossil form was a shorter and 

 proportionally a thicker fish than the living one. Its 

 pectoral fins were much larger and better developed. The 

 vertebral column in the two fishes offers a very close parallel. 

 In my specimen there are only twenty-two vertebrae preserved 

 behind the posterior dorsal spine ; but Sir P. Egerton 

 mentions a specimen, in the collection of the Earl of Ennis- 



32* 



