66 



FISHES. 



There is no trace of ribs or limbs. 



[J. Mliller, Vergleiohende Anatomie der Myxinoiden. Erster Theil. 

 Osteologie und Myologie, in Abhandl. Ak. "Wiss. Berlin, 1835.] 



The Chondropterygians exhibit a most extraordiaary diver- 

 sity in the development of their vertebral column; almost 

 every degree of ossification, from a notochord without a trace 

 of annular structure to a series of completely ossified vertebrae 

 being found ia this order. Sharks, in which the noto- 

 chord is persistent, are the ffolocephali (if they be reckoned 

 to this .order, and the genera Notidanus and JEcMnorhinus). 

 Among the first, Chimcera monstrosa begias to show traces of 

 segmentation ; but they are limited to the outer sheath of the 

 notochord, in which slender subossified rings appear. In 

 Notidanus membranous septa, with a central vacuity, cross 

 the substance of the gelatinous notochord. In the other 

 Sharks the segmentation is complete, each vertebra having a 

 deep conical excavation in front and behind, with a central 



Fig. 31. — Heterocercal Tail of Centrina salviani. 

 ci, Vertebrae ; 6, Neurapophyses ; c, Hiemapophyses. 



canal through which the notochord is continued; but the 

 degree in which the primitive cartilage is replaced by con- 



