BRITISH FOSSILS. 



Decade IX. Plate II. 



THEISSONOTUS. GeIs. Noy. 



[THRISSONOTUS. Agassiz. (Sub-kingdom Vertebrata. Class Pisces. Order 

 Goniolepidoti. Family Sauroidei. Sub -family Sauroidei homoeerci. 1st Group, tail 

 forked.) Body elongated ; dorsal fin opposite the interspace between the ventral and 

 anal fins ; anal fin extended ; scales rhomboidal, small, ganoid.] 



Thrissonotus Colei. Agassiz. Poiss. Foss. vol. 2, part 2, p. 128. 



The tapering head, gracefully elongated body, and well propor- 

 tioned fins combine to render this the most elegant of the sauroid 

 fishes of the liassic period. The specimen, which is unique, was 

 discovered many years since, and is in the possession of Lord 

 Enniskillen. The generic and specific appellations were given by 

 Professor Agassiz, who alludes to tliis specimen in the following 

 paragraph: — "The genus Thrissonotus is in some degree inter- 

 mediate between the genera Sauropsis and Thrissops. In general 

 appearance it also resembles the Pachycoroni, but it is decidedly 

 a new type, for the dorsal fin is situated in the middle of the back, 

 and yet it has the extended anal fin of a Thrissops." With reference 

 to these afiiliations, I may remark, in corroboration of its generic 

 distinction, that the body is more elongated than in the Pachy- 

 cormi, the dorsal fin more distal, and the scales entirely dissimilar 

 both from that genus and Thrissops. The extended anal fin re- 

 sembles that of Sauropsis and Thrissops, but the dorsal fin is more 

 advanced than in either of these genera, corresponding more nearly 

 in this respect with the dorsal fin of the genus Oxygnathus. The 

 scales have the nearest resemblance to those of the latter genus and 

 Cosmolepis, but are devoid of the surface ornament common to 

 those genera. 



[ix. ii.] 9 c 



