382 PALEONTOLOGY OF ILLINOIS. 



Gestus CTENOPTYCHIUS, Agassiz. 



Ctenoptychixjs peetenuis, St. J. and W. 



PI. Xa, Fig. 27. 



Teeth very small and fragile. Crown very compressed, crest rather 

 strongly arched, slightly acuminate, the apex nearly central, subacute, 

 minutely serrated, the denticulations more or less regularly increasing 

 in size from the lateral extremities where they are nearly obsolete, 

 tumid, subacute or rounded and separated by very short, faint sulci, 

 which appear in both faces in the perfect condition of the crown ; con- 

 cave face oval in outline, gently depressed vertically, nearly straight 

 laterally, basal margin broadly and more or less regularly arched down- 

 ward, or slightly sinuous in curvature, aud a little more rapidly conver- 

 ging towards one or other extremity, the opposite end less acute or more 

 rounded, coronal belt moderately prominent, narrow, consisting of at 

 least two imbrications ; convex face subcrescentiform, very slightly 

 arched in either direction, or nearly plane, basal angle slightly though 

 irregularly arched upward in the middle, and limited by a single narrow 

 coronal fold, which generally, perhaps always, terminates in the crest 

 on one or other side before reaching the extreme lateral angle, giving 

 to this aspect of the tooth the eccentric, unsymmetrical contour observed 

 in all the representatives of the form ; both faces apparently smooth, 

 highly polished. Base proportionally thick and strong, low, oblique, 

 lateral diameter slightly less than that of the crown, from which it is 

 strongly defined in both sides, inferior surface nearly in the same hori- 

 zontal plane as the crown, broad and well-defined along both margins, 

 gradually narrowed toward one extremity in the same direction as the 

 crown, as also noticable in the moderately channeled region beneath the 

 convex crown face, which occupies about one-third of the entire vertical 

 diameter of that side of the tooth, surfaces more or less roughened. 

 Lateral diameter of a small perfect tooth .16 inch, hight .13, depth of 

 convex face of base .03, or nearly equal to the hight of the opposite 

 crown-face, elevation of concave crown-face .10, or slightly greater than 

 the hight of the deep side of the base, breadth of inferior surface of 

 base .05 inch. A large specimen measures in lateral diameter .24 inch, 

 vertical diameter .20, showing a proportionately greater elevation than 

 in the preceding example. 



The form under consideration is represented in our collections by only 

 three or four examples, fortunately in a good state of preservation, 

 plainly exhibiting their distinctive characteristics. There seems to be 

 the most intimate relationship between these teeth and C. apicalls, 



