96 On the Skeleton of a Chimceroid Fish (Iscliyodus). 



end of the comparatively slender exserted portion is laterally 

 compressed, though apparently expanding again at the distal 

 end, where it is much broken. The denticles originally 

 clustered upon this spine are very slender, pointed, sigraoidally 

 bent, and fixed upon expanded bases. 



The vertebral column consists, as usual, of a closely 

 arranged series of delicate calcified rings, of which five in the 

 abdominal region occupy a length of 0'0035 and measure 

 0"004 in vertical diameter. 



Of the appendicular skeleton both the pectoral and pelvic 

 arches are too imperfectly preserved for description; but the 

 elongated claspers are faintly shown, and these do not appear 

 to have been provided with dermal booklets or spines. A 

 single denticle resting upon the pelvic cartilage may well have 

 been displaced from the group on the head. 



The dorsal fin-spine, which measures 0057 in length, is 

 remarkably slender and only slightly arched. The small 

 supporting cartilage is conspicuous at its base. In form and 

 proportions, and even in the restricted anterior area of the 

 superficial striations, it agrees precisely with the small spines 

 from the Stonesfield Slate described as Leptacanthus semi- 

 strlatus*^ and, if found at a Lower Oolitic horizon, would 

 be thus named without hesitation. In Elasmobrancii and 

 Chimgeroid fishes, however, the characters of the dorsal fin- 

 spines are often unreliable and insufficient for specific, or even 

 generic, determinations. 



No traces of calcified rings in the " lateral line " system or 

 of dermal tubercles are exiiibitcd ; but the absence at least of 

 the former is probably due to their lo^s in the extrication of 

 the fossil from the matrix. 



In conclusion, the Oxfordian fossil now described tends 

 further to confirm the reference of the Ischyodus-\\kQ fishes to 

 the existing family of Chimajridtc, and a peculiar form of 

 ^^Lejitacant/nts,''^ already assumed on theoretical grounds to 

 pertain to Ganodus f, is definitely proved to be at least 

 Chiniteroid. The impossibility of observing the oral surface 

 of the dental plates prevents, as already remarked, any satis- 

 factory determination ; but the external aspect of the den- 

 tition so closely resembles that of the well-known Upper- 

 Jurassic species Ischyodus EgertoniX that, until furtlier 

 evidence is discovered, the Christian Malford fossil may be 

 provisionally quoted as an immature example of that form. 



* L. Afrast^iz, Rccli. Poiss. Foss. vol. iii. (1837), p. 1*8, pi. vii. tijjrs. 3-8. 

 t WoiHhvaiil and Sl'.orbuni, ("at. Ihit. Foss. Vertobnvta (1890), p. 114. 

 \ L. Agab:<iz, to»i. cit. p. 340, pi. .\1. c. tigs. 1-10. 



