VERTEBRATES. 359 



boniferous period, and by far the most striking are furnished by the Coal 

 Measure strata. The corrugated and highly ornamented surface of these teeth 

 will serve to distinguish them at a glance from all the teeth of Orodus de- 

 scribed, therefore no extended comparisons are necessary. It should be said, 

 however, that this group of eleven teeth, though evidently belonging to one in- 

 dividual, give but an imperfect idea of his complete dentition. They are not 

 in their normal position, for the longer diameter of all doubtless once coincided 

 with the arch of the jaw on which they were placed. If this is true and the 

 analogies presented by the dentition of living and fossil cestracionts all indi- 

 cate that it is so we must suppose that the smaller teeth have been shifted 

 from their original position, in which they must have presented their ends 

 rather than their sides to the series of larger teeth. In Acrodus, the Jurassic 

 representative of Orodus, as in the living Cestracion, we find the largest and 

 strongest teeth placed near the middle of each mandible at the point where the 

 muscles which raise it act with the greatest mechanical advantage : the syin- 

 physis of the jaw being generally covered by more or less conical and what may 

 be called prehensile teeth ; while the posterior portion of each ramus bears rows 

 of teeth diminishing in size backward. A similar structure is visible in the 

 dentition of the mammalia, where the anterior portion of the jaw is occupied 

 by incisors and canine teeth, the middle portion by the molars, or grinders as 

 they are properly denominated. Reasoning from these analogies, we should con- 

 sider the group of teeth under consideration as having occupied the middle and 

 posterior portion of the left mandible or the right maxillary. 



What were the forms of the anterior teeth of the series to which these be. 

 longed we can only conjecture, but the very striking resemblance which the 

 teeth we have described under the name of Lophodus varialilis (PI. iv, fig. 4,4a, 

 46, 5, 5a, 11, Ha, 116, present to these, in the surface markings of all, and the 

 form of a part, suggest that the two groups once formed but portions of the 

 dentition of oiie genus; the conical forms of Lophodus being the anterior teeth. 



Formation and locality : Coal Measures; Alton, 111. 



GENUS HELODUS. 

 HELODUS RUGOSUS, N. and W. 



PI. ii, fig 10, 10a. 



TEETH small but strong ; crown broadly conical in outline, 

 set obliquely on the root, lateral extremities rounded, entire 

 surface roughened by papillae of enamel, or vermicular raised 

 lines ; root as high as the crown and nearly as wide above, 



