66 



FISHES. 



There is no trace of ribs or Kmbs. 



[J. Miiller, Vergleichende Anatomie der Myxinoiden. Erster Tlieil. 

 Osteologie uiid Myologie, in Abhandl. Ak. Wiss. Berlin, 1835.] 



The Chondroptcrygians exhibit a most extraordinary 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 in this order. Sharks, in which the noto- 

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

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

 Among the first, Chimcera monstrosa begins 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 Ceutrina salviani. 

 a. Vertebrae ; h, Neurajiopliyses ; c, Hfemapophyses. 



canal through which the notochord is continued ; but the 

 degree in which the primitive cartilage is replaced by con- 



