202 PISCES. 



vertebrae, and opposed to each other; but the gelatinous column 

 with its fibrous sheath that occurs in the Sturgeons and Chimeerae, 

 and also in the Cyclostomi (as will be described further on), has a 

 cellular structure, like the chorda dorsalis, quite different from car- 

 tilage. 



In the Plagiostomi the formation is of a more perfect kind, and 

 similar to that of the Osseous Fishes, for the upper and lower pieces 

 of the vertebrae having become more complete, the gelatinous column 

 is so enclosed that the conical facets of the vertebral bodies are 

 alone left. The points of ossification in its interior are very com- 

 plex. In some Rays and Sharks a hyaline cartilage rests upon the 

 surface of the vertebral body (e. g. Spinax, Scyllium) ; in others 

 the vertebral body, with cellular interstices, ossifies up to the sur- 

 face, but in its interior there remains a cross of hyaline cartilage, the 

 crura of which are directed toward the points of origin of the arches 

 and transverse processes (e. g. Carcharias, Zygaena) ; or other varieties 

 occur, as in many genera, e. g. Hexanchus, Heptanchus, where the 

 whole vertebral column remains cartilaginous throughout life. As 

 a rule, however, the vertebral bodies, the pieces forming the arches, 

 the intercrural cartilages, and the laminae forming the roof of the 

 spinal canal, are always to be distinguished. In the Rays a large 

 anterior portion of the vertebral column is not distinctly divided into 

 vertebral pieces, these being blended together. 



In Petromyzon the fibro -cartilaginous tube is found to be annu- 

 lated, filled with gelatine, and surrounded by a fibrous tunic, which 

 forms above it a tube for the spinal marrow. In the upper mem- 

 branous tube cartilaginous crura are seen to arise, and may be 

 viewed as rudiments of vertebral arches. 



In Myxine, Ammocaetes, and Bdellostoma, we meet with the 

 lowest persistent condition of the vertebral column, and one which 

 disappears at a very early period of existence in the higher Verte- 

 brata ; this consists of a chorda dorsalis, filled with gelatine, and 

 surrounded by a fibrous tunic that forms above a tube for the spinal 

 marrow ; but all special divisions or rudiments of ossification in this 

 tube are wanting. A similar chorda dorsalis, projecting as far as 

 the snout, together with a fibro-membranous capsule for the spinal 

 marrow, is found also in Amphioxus. Even in the Lepidosiren the 

 vertebral column consists of a mere chorda dorsalis, without any 

 indication of vertebral rudiments, and provided only with a liga- 

 mentous capsule and gelatinous substance. 



Cartilaginous accessory spines occur in the dorsal and anal fins 



