FISHES. 15 



vertebras more or less ossified are present. These present in front 

 and behind a conical cavity which is filled with a soft matter, the 

 remains of the gelatinous cord. These two cavities often communi- 

 cate in the middle of the body of the vertebras by a small aperture 

 between them, and thus have the form of an hour-glass placed 

 horizontally. In Lepidosteus the remarkable exception occurs, that 

 the bodies of the vertebrae exhibit a hemispherical surface at their 

 anterior part, an articular head, and at the back part are con- 

 cave 1 . 



Above the body of the vertebrae is situated a process on each 

 side (neurapophysis OWEN). These processes are ossified earlier 

 and more generally than the bodies of the vertebrae ; they approach 

 each other above, and thus form a ring above each of the vertebrae, 

 the superior vertebral arch, in which the spinal marrow is lodged, 

 and on which a spinous process is usually affixed. At the base of 

 this superior vertebral arch there is generally situated forwards and 

 backwards, sometimes forwards alone, a small articular process on 

 each side, which serves, with the conical cavities of the bodies of 

 the vertebrae, to connect the vertebras ; the anterior articular process 

 overlaps the posterior process of the preceding vertebra, and is 

 received in an excisure at the base of the superior vertebral arch. 

 Sometimes, as in Polypterus, the superior arches of the bodies of 

 the vertebrae remain distinct, but in most fishes they sooner or 

 later coalesce with them. The dorsal vertebrae have transverse 

 processes (parapopliyses OWEN) ; the caudal vertebrae processes 

 which bend downwards and approach each other to form in this 

 way an inferior vertebral arch, within which the trunks of the 

 blood-vessels are situated that run beneath the bodies of the caudal 

 vertebras. The number of vertebrae is very different in different 

 fishes ; in Orthagoriscus mola, for instance, there are only seven- 

 teen, in Anarrhichas lupus seventy-six, in Murcena more than an 

 hundred. The tail is formed by nearly the half, and often by many 

 more than the half, of these vertebrae. For the tail is in fishes the 

 chief instrument of motion, and in most of them constitutes the 

 greatest part of the body ; all the organs are pushed forward to 

 leave room behind for the large muscular mass which serves for 

 motion. 



1 AGASSIZ Reck, sur les Poiss. fos&iles, n. 2, p. 23, Tab. /3', figs. 10 12. A similar 

 disposition is found in most of the Reptilia, but in them the articular head is behind. 



