360 PALiE ONTOLOGY OF ILLINOIS. 



which latter form the present teeth may possibly prove to be identical. 

 But, taking into consideration the distinctive features preseuted by the 

 above described teeth, it would be mere assumption to recognize such, 

 specific identity in the present state of knowledge regarding their varia- 

 bleness, and the absence of material showing the gradations such as 

 have been illustrated by the large series of specimens of G. inconstans, 

 whose association render their evidence almost conclusive, and which 

 are equally strongly marked from the few examples of the present form 

 occurring at the same locality. The teeth under consideration present 

 some individual variation in size and form, though more to the extent 

 noticed in the above mentioned form. They approach G. insignis in the 

 outline of the convex aspect, but are distinguishable by their stronger 

 proportions and more angular general figure. 



Position and locality: Upper beds of the St. Louis limestone; Alton, 

 Illinois, St. Louis, jjlissouri, and Pella, Iowa. 



Chomatodxts inconstans, St. J. and W. 



PI. X, Fig. 5-14. 



The collections from a single locality and stratum, in the St. Louis 

 limestone of Iowa, afford a fine series of isolated teeth, which present 

 considerable variation in form and size, constituting three or four quite 

 well marked varieties, but which are intimately united by the gradation 

 of characters common to all, and which seem to furnish conclusive 

 evidence of their specific identity, however widely extreme examples 

 may appear to differ from one another. 1. The teeth here designated 

 possess in common the same coronal characters, which undergo the 

 least modification of any part of the tooth in the mutations which are 

 traceable in the various individuals which depart most widely from the 

 normal condition of the species. But in the basal region occur the 

 principal variations, which give rise to the more or less strongly marked 

 vertical forms herein noticed. The crown forms a rather stout, regularly 

 sloping ridge, the crest of which is subcentral or nearest the convex 

 border, which latter is gently arched in both directions, while the oppo- 

 site face is occupied by a wide and usually deeply excavated furrow 

 extending the entire length of the concave face, and margined below 

 by the thin, straight or slightly ardhed border, the upper surface of 

 which often presents a delicately wrought basal band composed of 

 several imbrications ; the crest of the crown usually culminates near 

 one or other extremity, whence on the one side the slope is abrupt to 

 the rounded or somewhat obliquely truncated extremity, while on the 

 other it gradually diminshes in elevation towards the obtusely pointed 

 extremity of the tooth ; the basal band, instead of forming a narrow 



