168 



EEPTILIA 



The slender rami of the lower jaw unite in an extensive symph^^sis, and 

 are without a coronoid process. Each ramus consists of five or sometimes six 

 pieces. A deep groove extends along the outer side of the dentary on its 

 upper border, but becomes interrupted anteriorly into a series of pits for the 

 blood-vessels. 



The teeth (Fig. 265) are acutely conical, smooth or vertically striate, some- 

 times with anterior and posterior carinae, and with a tumid root usually 

 larger than the crown. As many as 180-200 occur in single series in upper 

 and lower jaws (Fig. 266), being confined to the maxilla, premaxilla, and 

 dentary. They are placed in a deep groove, usually continuous, and were 



Na 



Pmx 



Fig. 265. 



Ichthyosaurus platyodon, Conyb. Lower 

 Lias ; Lyme Regis, England. Tooth, lateral 

 and anterior aspects, i/i (after Lydekker). 



Fig. 266. 



Ichthyosaurus quenstedti., Zittel. Upper Jura ; 

 Wiirtemberg. Jaw-fragment, 3/_j. {ind, Dentary ; 

 na, Nasal ; pnu:, Premaxilla), (after Quenstedt). 



held in place by the tissues and edges of the groove. Some of the later 

 Mesozoic forms are edentulous. The crown is composed of dentine, covered 

 with a layer of enamel, and over this there may be sometimes still another 

 layer of cement. A vertical folding of the walls similar to that occurring in 

 Labyrinthodonts is occasionally observed. The large root is composed of an 

 osseous cement containing bone cells and vascular canals, and is enveloped by 

 a more or less plicated covering of dentine. 



The vertebral column comprises but two regions— caudal and precaudal. 

 There is no true neck, and no sacrum. There are between 120 and 150 

 vertebrae, of which about 100 are caudal. The centra are extremely short, 

 and deeply biconcave, as among Selachians and Labyrinthodonts. The neural 

 arches are strongly developed, never fused with their centra, and articulate 

 with one another by feeble zygapophyses. Adult individuals almost invariably 

 have the atlas and axis fused, and before and behind each of them are sub- 

 vertebral hypocentra or intercentra. The remaining precaudal vertebrae are 

 of nearly uniform character (Fig. 267), each centrum being provided with two 

 pairs of tubercles for attachment with the double-headed ribs, and exhibiting 

 dorsally a neural canal, with a rugose pit on either side for the pedicles of 



