308 VERTEBRATA. 



authority as Professor Agassiz, is founded solely upon the 

 nature of the integumentary covering. The palaeontologist, 

 however, whose materials often consist of nothing more than 

 detached scales, is not rarely driven, by the necessity of the 

 case, to provisionally classify his specimens in accordance with 

 the nature of these appendages. 



As regards their true osseous system or endoskeleton, Fishes 

 vary very widely. In the Lancelet there can hardly be said 

 to be any skeleton, the spinal cord being simply supported by 

 the gelatinous notochord, which remains throughout life. In 

 others the skeleton remains permanently cartilaginous ; in 

 others it is partially cartilaginous and partially ossified ; and, 

 lastly, in most modern fishes it is entirely ossified, or converted 

 into bone. Taking a bony fish (fig. 275) as in this respect a 

 typical example of the class, the following are the chief points 

 in the osteology of a fish which require notice : 



The vertebral column in a bony fish consists of vertebras 

 which are hollow at both ends, or biconcave, and are techni- 

 cally said to be " amphicoelous." The cup-like margins of the 

 vertebral bodies are united by ligaments, and the cavities 

 formed between contiguous vertebrae are filled with the gela- 

 tinous remains of the notochord. This elastic gelatinous sub- 

 stance acts as a kind of ball-and-socket joint between the 

 bodies of the vertebrae, thus giving the whole spine the extreme 

 mobility which is requisite for animals living in a watery 

 medium. The ossification of the vertebrae is often much more 

 imperfect than the above, but in no case except that of the 

 Bony Pike (Lepidosteus) is ossification carried to a greater 

 extent than this. In this fish, however, the vertebral column 

 is composed of " opisthoccelous " vertebrae that is, of vertebrae 

 the bodies of which are concave behind and convex in front. 

 The entire spinal column is divisible into not more than two 

 distinct regions, an abdominal and a caudal region. The ab- 

 dominal vertebrae possess a superior or neural arch (through 

 which passes the spinal cord), a superior spinous process 

 (neural spine), and two transverse processes to which the ribs 

 are usually attached. The caudal vertebrae (fig. 275) have no 

 marked transverse processes ; but in addition to the neural 

 arches and spines, they give off an inferior or /uzmal arch 

 below the body of the vertebrae, and the haemal arches carry 

 inferior spinous processes (haemal spines). 



The ribs of a bony fish are attached to the transverse pro- 

 cesses, or to the bodies, of the abdominal vertebrae, in the form 

 of slender curved bones which articulate with no more than 

 one vertebra each, and that only at a single point. Unlike the 



