MKRRIAM: TRIASSIC ICHTHTOSAURIA. 



Fig. 9. Cymbospondyhis petrinus Leidy. Inner side, posterior end of left ramus of mandible, X %. 

 Ar, articular; Cn, coronoid; An, angular; Sa, surangular; H, hyoid; Q, quadrate. 



tends from the eotylus to the posterior end of the mandible. It is much thick- 

 ened immediately behind the eotylus, and is thickened also at the posterior end. 

 Between these regions the upper surface is strongly hollowed. 



DENTITION. 



In the specimens of Cymbospondylus that have been examined thus far the 

 dentition has not been materially different from that of Ichthyosaurus except- 

 ing in the insertion of the teeth (figs. 10 to 12). The numerous teeth are all 



10 



-Sp 



12 



11 



Fig. 10. Cymbospondyhu petrinus Leidy. Horizontal cross-section of dentary about 5 mm. below 



upper margin, showing teeth set in completely surrounded bone sockets, X Ms- 

 Figs. 11 and 12. Cymbospondylus petrinus Leidy. Fig. 11, transverse cross-section of lower jaw 

 showing teeth set in sockets in the dentary. Superior margin cut down several millimeters. T, 

 tooth ; D, dentary ; Sp, splenial, X % ; fig. 12, anteroposterior cross-section of the base of a tooth 

 set in the dentary, X %. 



simple conical, and are, in some cases at least, set in distinct pits. The 

 base of the dentine cone is strongly folded (fig. 10), and there does not 

 appear to be any considerable amount of cement covering it. As nearly as 

 can be determined there were thirty to thirty-five, or more, teeth in each ramus 

 of the jaws. Between the anterior and posterior ends of the jaws there is a 

 slight difference in form, the posterior teeth being somewhat shorter and heav- 

 ier (fig. 3). This is also true of the genus Merriamia (fig. 8), of which the 



