122 



THE ANATOMY OF VERTEBRATED ANIMALS. 



whicli not only overlap, but are fitted together by pegs and 

 Bockets, where their anterior and posterior edges come into 

 contact. 



Fig. 38. — The brain of Lepidosteiis semiradiatus. A. From above; B. From below:/ 

 the medulla oblongata ; (?, the cerebellum ; c, the oi)tic lobes of the mesencephalon ; y, 

 the cerebral hemispheres ; A, the pituitary bcdy ; «, the lobi inferiores. Ch, the chiaa- 

 ma ; I., olfactory ; II., optic nerves. 



The endoskeleton is not less diversely modified ; and it is 

 worthy of remark that no sort of relation, either direct or 

 inverse, is traceable between the completeness of the endo- 

 skeleton and that of the exoskeleton. Thus Spatularia^ 

 Scapirhynchus^ and Accipenser have a persistent notochord, 

 in the sheath of which mere cartilaginous rudiments of the 

 arches of vertebras appear. The ribs, when present, are par- 

 tially ossified. Polypterus and Amia have fully ossified ver- 

 tebrifi, the centra of which are amphiccelous. Lepklosteus also 

 has fully-ossified vertebra? ; but their centra are opisthocoe- 

 lous, having a convexity in front and a concavity behind, as in 

 some Amphibia. 



More or fewer of the anterior vertebrae, or their cartilagi- 

 nous representatives, are united with one another, and ^vith 

 the posterior part of the skull. And the cranium may consist 

 principally of cartilage, membrane bones being superadded ; 

 or the primordial cartilage maybe largely superseded by bone, 

 as in the Teleostei, 



Spatularia^ Scapirhynchus^ and Accipenser^ have skulls of 

 the former description. The cranium is one mass of cartilage, 

 continuous behind with the coalesced anterior spinal cartilages, 

 so as to be immovably connected with the spinal column. The 

 notochord enters its base, and terminates in a point behind 

 the pituitary fossa. In front, the cartilage is produced into a 



