ENALIOSAURS. 281 



elevated longitudinal ridges, terminating abruptly at different dis- 

 tances from the apex, to which, however, none of them extend. 



In the lower jaw of the Pies, macrocephalus there are twenty-six 

 teeth on each premandibular bone : the crown of one of the large 

 anterior tusks, in a lower jaw, ten inches in length, measured one 

 inch and a half in length and one third of an inch in breadth ; its 

 transverse section was nearly circular. The premandibular piece 

 of the lower jaw of the Pies, arcuatus contains fifty-four teeth, or 

 twenty-seven on each side, the six anterior teeth on each side being 

 larger and longer than the rest. In the lower jaw of the Pies, 

 dolichodeirus there are twenty-five teeth on each side, the four anterior 

 of which are the largest. 



The disposition of the calcigerous tubes is shown in a longitu- 

 dinal section of the apical third part of a tooth of the Plesiosaurus 

 Hawkinsii in PL 74. In their general course they bear a considerable 

 resemblance to the same parts in the Ichthyosaurus, but the primary 

 cuievature presents a more graceful sigmoid line, from the inclination of 

 the peripheral extremities of the tubes towards the apex. The diameter 

 of the tubes near their origin is jMooth of an inch; their interspaces equal 

 five or six of their diameters ; the secondary undulations are relatively 

 wider than in the Ichthyosaurus, and the secondary branches are longer 

 and more bent ; the tubes divide dichotomously several times in 

 their course ; the divisions, after a slight divergence proceeding in the 

 same parallel line with each other and with the main stem. The finer 

 secondary branches dilate into extremely minute cells along tracts 

 which run parallel with the contour of the tooth itself, and occasion 

 the apparent alternation of opaque and clear layers observable in the 

 section by transmitted light, as represented in the figure ; the mode 

 of termination of the calcigerous tubes is shown at fig. 2, PI. 74. 

 The enamel presents the same fine fibrous structure as in the 

 Ichthyosaurus. 



The mode of succession of the teeth of the Plesiosaur differs 

 from that of the Ichthyosaur in the growing tooth being developed 

 in a cell at the inner side of the old socket, and affecting by its 

 pressure the bone of the jaw, rather than the tooth about to be 

 displaced. 



