YERTEBEATES. 341 



Desjiiodus costellipertjs, St. J. and W. 



PI. Xa, Fig. 10,11. 



Teeth minute, strong, more or less symmetrical. Crown more or less 

 strongly arched between the extremities, long elliptical in outline, 

 obtusely rounded at the extremities, crest sharply denned, obtusely 

 angular and gradually ascending from the extremities towards the mid- 

 dle, where it is more or less suddenly produced into a strong subcorneal, 

 more or less eccentric median prominence, which is strongly produced 

 iu front forming a sharp vertical buttress, which is crossed by one or 

 two delicate lateral carinse, giving rise in the lower portion of the angu- 

 lation to as many small node-like prominences of which the inferior one 

 is largest; posterior face moderately arched laterally and vertically, 

 sometimes with a broad convexity in the region of the median promi- 

 nence, anterior face abrupt, gently concave in the lateral portions, but 

 abruptly and sharply produced in the median buttress ; basal borders 

 strongly iubeveled, broadly arched upward in the middle and sharply 

 so at the produced extremities, coronal fold wide, stronglydefined along 

 the obtuse angle of junction with the superior coronal region, the upper 

 edge slightly overlapping and delicately striated vertically ; the coronal 

 surfaces are polished and smooth in the lower portion, along the crest 

 sharp vertical costse appear, which culminate in the delicate denticula- 

 tions, of which latter there occur four to seven, more or less, in either 

 one or other extremity, in the abrupt lateral declivity of the median 

 prominence still fainter thread-like lines are observed, and in the more 

 or less tumid convex face a few stronger costae converge in the apex of 

 the median prominence, sometimes only a single ridge occurs, forming 

 a delicate vertical carina descending half way to the coronal fold. Base 

 strong, nearly vertical to the crown, which it exceeds in depth, though 

 considerably less in lateral and anteroposterior diameter, posterior face 

 rectangular in outline, slightly arched laterally and faintly depressed 

 vertically in the middle, with a faint fold at the base of the inbeveled 

 coronal border, and slightly beveled along the inferior margin, anterior 

 face moderately excavated, superior shoulder prominent, with a rela- 

 tively strong basal protuberance beneath the median angulation, infe- 

 rior surface well-defined, and very obliquely beveled to the posterior 

 edge ; basal surfaces coarsely roughened and pitted. The teeth seldom 

 exceed .15 inch in lateral diameter. 



Of the present form perhaps two more or less well-marked varieties 

 are noticable, as represented by laterally elongated teeth, on the one 

 hand, and on the other laterally abbreviated, stouter teeth, in which the 

 median prominence is relatively stronger and the lateral portions of the 



