66 THE VERTEBRATE SKELETON. 



though in the neural arches this segmentation is readily trace- 

 able. The neural arches of the first few vertebrae are fused 

 together and completely surround the notochord, while they 

 do not in other parts of the body. The tail is diphy cereal. 

 Of the living genera, in Callorhynchus there is no trace of 

 calcification in the skeletogenous layer, while in Chimaera rings 

 of calcification are found, there being three to five for each 

 vertebra as indicated by the foramina for the exit of the spinal 

 nerves. The pelvic fins are produced into claspers. Besides 

 the living genera Chimaera, Harriotta and Callorhynchus a 

 fair number of fossil forms are known, e.g. Ischyodus. 



Order 3. GANOIDEI. 



The fishes included under the term Ganoidei form a very 

 heterogeneous group, some of which closely approach the Dipnoi, 

 others the Elasmobranchii, others the Teleostei. The great 

 majority of them are extinct, only eight living genera being 

 known; these are all inhabitants of the northern hemisphere, 

 and with the exception of Acipenser, which is both fluviatile 

 and marine, are entirely confined to fresh water. 



The following is a list of the living genera of Ganoids with 

 their respective habitats : 



Acipenser. Rivers and seas of the northern hemisphere. 



Scaphirhynchus. Mississippi and rivers of Central Asia. 



Polyodon (Spatularia). Mississippi. 



Psepkurus. Yan-tse-kiang, and Hoangho. 



Polypterus. Rivers of tropical Africa. 



Calamoichthys. Some rivers of West Africa. 



Lepidosteus. Freshwaters of Central and North America 

 and Cuba. 



Amia. Rivers of Carolina. 



The exoskeleton is very variable, thus the body may be : 



(a) Naked or with minute stellate ossifications as in 



the Polyodontidae. (b) Partially covered with large detached 



