352 PALEONTOLOGY OF ILLINOIS. 



Position and locality : la the middle and upper beds of the St. Louis 

 limestone; Alton and Monroe county, Illinois ; St. Louis, Missouri. 



Venustodtts arctUtus, St. J. and W. 



PI. IX, Fig. 5, 6. 



Teeth of small size, gently arched aloug the convex border, more or 

 less arched vertically, extremities rounded or angularly truncated, dis- 

 tance in a direct line between the extremities of a medium-sized 

 specimen .32 inch, autero posterior breadth of crown .09 inch. Of this 

 form two more or less well-marked varieties may be recognized, the 

 smaller of which is distinguished by its more strongly arched outline, 

 and in' some instances faint sigmoidal curvature, obtusely rounded at 

 the extremities ; besides, the crown is centrally traversed by a low or 

 moderately elevated crest, which culminates in a strong, conical, 

 laterally deflected median cone, which in worn specimens is tumid, and 

 flanked in either extremity by a row of more or less prominent denticu- 

 lations, which regularly decrease in size on nearingthe extremities, and 

 are more strongly developed upon that side overshadowed by the eccen- 

 tric median cone, varying from eight to eleven in number ; the convex 

 surface is slightly arched vertically, often interrupted in the median 

 region by a tumid distension of the basal portion of the principal cone, 

 in hight little less or even exceeding that of the opposite face, which 

 latter presents a shallow depressed area, terminated below by a com- 

 paratively wide coronal belt composed of as many as three more or less 

 regular and strongly marked imbricating folds, which are reduced to 

 one or two narrow folds in the convex face. 



The second variety, which we have identified with the present species, 

 is characterized by its rectangular outline as seen from above, is less 

 strongly arched vertically, and traversed by a subcentral crest, which 

 rises more abruptly from the convex border, culminating in a somewhat 

 eccentric vertical cone, with from four to seven lateral denticulations 

 in either extremity ; convex face relatively low, lunate in outline, gently 

 arched in both directions, and margined below by two or three narrow 

 imbricating folds, which fold round the extremities coalescing with the 

 much wider belt of the opposite face; the concave face is about one- 

 third higher than the opposite side, of which fully one-half is embraced 

 in the numerously imbricated coronal belt, which forms a broad border 

 projecting outward and downward in a less abrupt descent than the 

 declivity of the superior surface, lateral extremities abruptly truncated 

 or but slightly rounded ; the thread-like imbricating plicae of the coro- 

 nal belt more or less frequently and irregularly anastomose, with inter- 

 calated folds in the stronger and more widely spaced upper portion of 



