VERTEBRATES. 291 



the intermediate spaces, both faces more or less strongly ridged verti- 

 cally, the outer face of the median cone often strongly buttressed and 

 descending to the basal protuberance ; coronal surfaces covered with a 

 polished enamel-like layer. 



The teeth of the several forms of the present genus at present known, 

 are small, or attain medium size. The individuals of certain forms are 

 very variable, and yet, when perfect, their minor or specific characteris- 

 tics are readily determinable. The affinities of the genus are clearly 

 intermediate between Orodus and Hybocladodus of the Carboniferous 

 genera, and its relation to the Jurassic Hybodus reticulatus of Agassiz, 

 would appear to be most intimate, but for the presence of the median 

 protuberance in the anterior face of the base. The general and some- 

 what marked resemblance to Cladodus, as especially displayed in the 

 denticulation of the lateral extremities, recurvation of the median 

 cusps, and the posterior expansion of the base of certain forms, suggest 

 a much more intimate relationship of the latter genus with Orodus 

 than has before been pointed out. The genus thus far is known only in 

 the lower members of the Lower Carboniferous, extending from the Kin- 

 derhook upwards to the Keokuk formation. 



Mesodmodtjs exsctjlptus, St. J. and W. 



PI. V, Fig. 18-22. 



Teeth attain medium size, though very variable, according to the posi- 

 tion they occupied upon the jaws. Perhaps three varieties may be 

 recognized, at least for the purpose of their specific definition, and 

 which are supposed to represent the dentition of the median, anterior 

 and posterior portions of the jaws, although these merge into one 

 another so that there is practically a regular gradation from one extreme 

 to the other. Of these the median teeth are the largest and most sym- 

 metrical, attaining nearly an inch in lateral diameter. They are gently 

 and regularly arched between the extremities, which are sometimes 

 curved forward, though generally directed backward, or nearly linear. 

 The base nearly or quite equals the crown in hight and lateral diame- 

 ter, exceeding the antero-posterior diameter of the crown, massive, 

 obliquely and moderately produced posteriorly, or nearly vertical to the 

 crown, posterior face gently convex vertically, anterior face beveled 

 from the Darrow shoulder to the broad inferior surface, which latter 

 occupies nearly the same horizonal plane as the crown; both basal 

 faces are more or less coarsely roughened, the anterior face more so 

 than the posterior, inferior surface smooth, or pitted with minute fora- 

 mena. Crown moderately convex and slightly compressed immediately 



