70 PALEONTOLOGY OF ILLINOIS. 



rounded with a slight sigmoidal curvature extending into the obscure 

 posterior alation, the vertical plane of the base equal in depth to 

 the height of the enamel fold from which it is well denned ; outer 

 margin obliquely truncated from the anterior angle downward and 

 outward, usually much worn and otherwise mutilated. The anterior 

 and posterior borders show more or less distinct and rather wide 

 undulations. Coronal surface uniformly and closely punctate, some- 

 what strongly arched transversely into a broad rounded prominence 

 culminating a little anterior to the median line, posterior slope 

 gently concave. Breadth of medium-sized tooth across inner mar- 

 gin, 23 mm. ; ditto, outer margin, 18 mm. ; length along antero- 

 lateral border, 20 mm. ; about equal to the opposite border ; depth 

 of tooth at middle of anterior border, 2-5 mm. Second, median 

 form of the lower (?) jaw: Teeth corresponding with the second 

 form, described under P. obliquus, and having the same general 

 outline, but distinguishable by the less rapid convergence of the 

 lateral borders and the considerable gibbosity of the coronal region, 

 which rises into a low broad prominence, traversing the crown 

 nearly centrally, or a little abaft the median line. The large uniqu 

 individual of this particular form afforded by the collections, is 

 somewhat mutilated, the borders and extremity not being clearly 

 shown ; the inner margin is broadly rounded, with a slight sigmoidal 

 oblique course, the crown surface punctate as in the other forms. 



Comparisons of the first above mentioned form with the type 

 specimen described by Messrs. Newberry and Worthen, under the 

 name Helodus placenta, leaves no doubt as to their specific identity. 

 The original specimen figured in the report of the Illinois Geol. 

 Survey, II, PI. V, fig. 4, 4a, is that of a small individual, the 

 transverse diameter of which considerably exceeds the length from 

 within outwards, the inner margin and base being in an extraor- 

 dinarily perfect state of preservation, even showing the delicate cren- 

 ulations of the rather heavy coronal fold that rises from and projects 

 somewhat beyond the mural basal area. The outer portion of the 

 tooth, on the other hand, is much worn, though the entire outline 

 is still preserved, its characteristics being well reproduced in one of 

 the figures illustrating the species as herein interpreted. 



Compared with Psephodus olliquus, the forms of the present species 

 reveal the following salient distinctions: The supposed mandibular 

 median teeth are relatively narrower, lateral borders less rapidly 

 converging, the more nearly central position of the axis of the 

 coronal ridge, and the less angularly rounded inner margin; in the 



