ON THE VERTEBRATE SKELETON. 



283 



The Teeth. 

 The bones of the dermo-skeleton are, — 

 The Supratemporals ; 

 The Supraorbitals ; 

 The Suborbitals ; 

 The Labials. 

 Such appears to be the natural classification of the parts which constitute 

 the complex skull of osseous fishes. 



As the object of the present report relates chiefly to the endoskeleton, I 

 have only added the osseous parts of the sense-capsules to the cranial vertebrae 

 in fig. 5 ; omitting the branchial arches and dermal bones : the haemal arches 

 and their appendages are given in diagrammatic. outline. 



Reptiles. — In proceeding with the inquiry into the natural arrangement of 

 the skull-bones, I have selected from the Reptilia the crocodile, as a typical 

 example of that class, and one most likely to facilitate the inquiry on account 

 of the characteristic persistence of the primitive cranial sutures. 



Pursuing the same mode of investigation as in the case of the fish, let us 

 disarticulate the hindmost segment of 



Fig. 18. 



Disarticulated epencephalic arch, viewed from 

 behind : Crocodile. 



the skull and so detach the four bones, 

 represented in fig. 18. The dotted 

 circle indicates the points at which 

 these bones are joined together, in 

 order to encompass the epencephalon, 

 or hindmost segment of" the brain. 

 No. l is the centrum ; 2, 2 are the neur- 

 apophyses with the coalesced par- 

 apophyses (4, 4) ; and 3 is the neural 

 spine. This element differs but little 

 in size and shape from the similarly 

 detached and depressed neural spine 

 of the atlas of the crocodile. The 

 single convex condyle at the back part 

 of no. 1 makes that centrum resemble 

 the posteriorly convex bodies of the 



trunk-vertebrae in as striking a manner as the repetition of the articular 

 concavity in the basioccipital of the cod (fig. 1, 1) marks its serial homo- 

 logy with the succeeding vertebral centrums of the same animal. In the 

 descending process from the under part of the occipital centrum of the 

 crocodile (fig. 18, 1), we see a second character of the cervical centrums in 

 that reptile repeated, viz. their inferior exogenous spine. The neurapo- 

 physes (2, 2), like those of the atlas, meet above the neural canal : they give 

 exit to the vagal and hypoglossal nerves, and protect the sides of the me- 

 dulla oblongata and cerebellum. The neural spine (3) protects the upper 

 surface of the cerebellum : it is also traversed by tympanic cells, and assists, 

 with the bones 2, 2, in the formation of the chamber for the internal ear. 

 The special homology of the outstanding processes (4, 4) in the crocodile 

 and serpent (fig. 10), with the similarly situated but distinct 'paroccipital' 

 bones in the cod, is confirmed by their resuming their independency in the 

 hinder segment of the skull of the chelonian reptiles • and the occipital neural 

 arch of the crocodile is reduced by their confluence with the neurapophyses 

 to the condition of those of the trunk-vertebrae, as composed, viz. of four 

 instead of six elements. 



The epencephalic arch offers the same simple condition not only in the 

 ophidians but in most saurians : the chameleons however retain, like the 



u 2 



