ME. J. W. DAVIS ON CHONDKOSTETJS ACIPENSEEOIDES. 



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41. On Ceoneeostees acipenseeoiees, Agassiz. 

 By James W. Davis, Esq., E.L.S., &c. (Read March 9, 1887.) 



[Plate XXIII.] 



Two species of Chondrosteus have been described by Sir Philip 

 Egerton, viz. Chondrosteus acipenseroidcs and C. crassior. Both 

 were obtained from the Lias of Lyme Regis. The former species 

 was known to Louis Agassiz, who named, hut did not de- 

 scribe it (' Poissons Fossiles,' vol. ii. part 2, p. 280). In 1858 

 Egerton communicated a paper to the Royal Society (Phil. Trans, 

 vol. xlviii. p. 871) on Chondrosteus, in which he described a number 

 of specimens, which he referred to the species named by Agassiz and 

 to the second one named above. The specimen which afforded the 

 most perfect description was in a slab of indurated shale. A part 

 of the head and the tail were missing ; the length of the parts 

 preserved was 15 inches, and the total length of the fish was esti- 

 mated at 24 inches. This, together with examples of the head of 

 C. acipenseroidcs, and a head and a good example of the tail of 

 C. crassior, afforded a large amount of detailed information, on which 

 Sir Philip Egerton was enabled to base the description and the deter- 

 mination of the genus. The characters of the species are indicated 

 as follows (loc. cit. p. 883) : — " The numerous specimens derived from 

 smaller individuals present indications of two species, one having 

 the bones of the head thin and smooth, the other being characterized 

 by stronger cranial plates, having a granulated exterior. The anal 

 fins of the latter are more massive, and have the transverse articu- 

 lations at shorter intervals. I propose to retain the Agassizian 

 name Chondrosteus acipenseroides for the former, and to designate 

 the latter as Chondrosteus crassior." In addition to these, mention 

 is also made of a third species, C. pachyurus, a large fish, estimated 

 at 5 feet in length ; only fragments were known, the tail being most 

 perfectly preserved. Of the tail, the most remarkable character is 

 " the excessive development of the scales bordering the upper margin. 

 The size and solidity of these scales is such that they more resemble 

 the broad teeth of an Acrodus than any dermal development with 

 which I am acquainted." The length of the tail is about 18 inches. 



Examples of the fossil fishes of the genus Chondrosteus, Agass., are 

 not unfrequently found in the Lias shales of Lyme Regis ; but it rarely 

 happens that a specimen is found so well preserved as the one it is 

 now proposed to describe (PI. XXIII.). The head (with the excep- 

 tion of the snout), the trunk, and the tail are exceptionally complete, 

 and during fossilization have maintained their proper relative 

 position. The bones remaining in their natural position afford 

 evidence, which has not hitherto been available, of the exact size 

 and relative proportions of the fish : and, what is of equal or still 

 greater importance, in addition to the osseous structure of the head 

 and that of the tail, a considerable portion of the elements of the 

 vertebral column is preserved. The dorsal and pectoral fins are 



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