16 VERTEBRATA phylum v hi 



the Ganoids Polyptei us alone retains two small pieces of cartilage as remnants 

 of a pelvic girdle ; in all other ganoids, as well as in the Teleostei, the pelvic 

 girdle is completely wanting, but the metapterygium of each side attains a 

 remarkable size, and forms the whole fin support, sometimes as a simple 

 elongated bone, sometimes as a plate forking into two pieces. In the Ganoidei 

 the rays belonging to the fin skeleton are occasionally bony ; in the Teleostei, 

 on the other hand, they are usually cartilaginous, much reduced, sometimes 

 indeed completely absent. The membrane of the fin itself comprises numer- 

 ous articulated bony rays. In the Dipnoi the pelvic girdle exhibits a 

 peculiar development. It consists of an unpaired, quadrangular plate of 

 cartilage, which is produced forwards into a long process, and supports the 

 cartilaginous axis of the biserial fin. 



While the pelvic fins of the Selachii, Ganoidei, and Dipnoi are always 

 situated near the hinder end of the abdominal cavity, they are often displaced 

 forwards in the Teleostei, becoming even connected with the pectoral girdle. 

 Through this forward displacement of the pelvic fins there arise striking 

 modifications in the general aspect of the bony fishes, which are of great 

 importance as a means of classification. 



The fossil bones of fishes can generally be recognised with certainty by 

 their external form. Their outer surface usually exhibits a characteristic 



aspect, on account of numerous small rugosities, open- 

 ings of canals, and striatums ; while tubular bones with 

 a medullary cavity are here entirely wanting. From 

 the histological point of view, the bones of many fishes 

 are noteworthy for the absence or very sparse develop- 

 fig. 29. ment of bone cells. 



Ear bones (otoliths) of a -r< -i i 1 Tii /-n- nrw i 



Teieostean fish, outer (A), and dossil ear bones or otoliths {rig. 29) are also some- 



LTttdoff.' as S tS size° lig0cene ; tim es found with the hard dermal structures and the 



skeleton. These do not consist of phosphate, but of 

 carbonate of lime, and exhibit great variety in their shape, the sculpturing of 

 their surface, and the nature of their borders. 



Since the work of Aristotle, the classification of fishes has been based 

 chiefly on the characters of the internal and external skeleton, on the nature 

 and arrangement of the fins, and on the organs of respiration. Artedi, 

 Bonaparte, Lacepede, Cuvier, and Valenciennes distinguished the cartilagin- 

 ous fishes from those provided with a bony skeleton, and again subdivided 

 both sections in different ways into several groups. In this classification no 

 reference was made to fossil fishes, although sharks' teeth, for example, teeth 

 of various bony fishes (the so-called bufonites, chelonites, toad-stones, and 

 snakes' eyes), and also complete skeletons from different localities (e.g. 

 Eisleben, Solenhofen, Oeningen, Monte Bolca, and Glarus) were already 

 known in the eighteenth century and numbers of them had been described. 

 The epoch-making work of L. Agassiz (1833-1844) afforded a complete survey 

 of all the fossil fishes known at the time, and also established on scale 

 characters a new subdivision of the fishes into four principal groups, namely, 

 Placoidei, Ganoidei, Cycloidei, and Ctenoidei. Johannes Miiller united the two 

 latter groups under the denomination of Teleostei (bony fishes), defined the 

 ganoids more precisely, and subdivided the cartilaginous fishes into four 

 sub- classes, namely, Leptocardii, Cydostomi, Selachii, and Dipnoi. The most 

 important of the later changes in classification consist in the establishment 



