34 



BRITISH FOSSILS, 



of ordinary osseous fishes, in which it is usually connected with the 

 skull only by ligament. The Siluroids and Ganoids, however, 

 coincide in always having this bone more closely united with the 

 regular cranial bones, and Coccosteiis, it will be observed, agrees 

 with them. 



Fig. 20. 



Arius rita, after Cuvier and Valenciennes, 



So much for the cranial shield. To comprehend the dorsal and 

 ventral body shields we must have recourse, not to Clarias, but to 

 other Siluroids, such as Bagrvbs, Arius, &c. In these fishes, in fact, 

 the anterior dorsal interspinous bones become so modified as to 

 form a great shield-shaped dermal plate, with a strong inferior crest, 

 which occupies the same position and has the same relations as the 

 medio-dorsal plate of Coccosteus, though it commonly bears a strongly 

 articulated spine, which is absent in the latter genus. In some 

 species, as Arius cruciger, the principal plate is provided with 

 lateral accessory plates, in which, perhaps, we have the homologues 

 of the dermal plates &, of Coccosteus. It is possible that c may 

 have been the operculum, which occupies a nearly similar position 

 in Arius, but if it were suturally connected with the supra- 

 scapula, this view would be untenable, and the bone would have 

 to be regarded as a scapular element. 



In the Siluroids to which I have referred, and in Loricaria, a vast 

 latero-ventral shield is produced by the prodigious expansion and 

 coalescence of the bony elements which are homologous with those 

 termed " coracoid " and radius in other fishes. Viewed from 

 the ventral surface, these bones form four great plates, those 



