VERTEBEATES. 165 



The dozen or so examples of each of the above described forms 

 contained in the collections, offer facilities for acquiring a somewhat 

 comprehensive understanding of their characteristic features, which 

 are remarkable for their persistency, the individuals presenting com- 

 paratively slight variation among themselves. Although associated 

 in the same strata, in every instance thus far observed the teeth 

 occur as isolated specimens disconnected from their original position 

 in relation to one another. Therefore, in regard to the assumed 

 specific identity of the forms here alluded to, the presumptive evi- 

 dence is precisely the same as allowed in the association of forms 

 in other specific groups, the justification of which is derived from 

 what is known in relation to the diverse dentition of Cochliodus. In 

 the present species, however, we are as yet unable to determine the 

 character of the teeth holding anterior positions upon the jaws. In 

 the case of the mandibular terminal or posterior teeth, the relatively 

 short articular border necessitates a corresponding abbreviated longi- 

 tudinal diameter for the tooth immediately in advance ; and so, also, 

 as regards the maxillary teeth, which latter may have been associ- 

 ated with teeth whose form bears the remotest suggestion of their 

 relationship with the teeth described. 



Geological position and locality : Chester limestone, Chester ; and 

 near Columbia, Monroe county, Illinois. 



DELTODOPSIS? CONVOLUTUS, St. J. and W. 



PL XI, Fig. 11-12. 



Teeth obliquely subtrigonal in outline, strongly arched from within 

 outward, with strong spiral inrollment. Antero-lateral border making 

 a gentle sigmoidal curvature in its slightly oblique forward and out- 

 ward course, basal portion very deep, broadly channeled, terminat- 

 ing below in a narrow rim, and bordered above by the rounded, 

 inbeveled enamel fold which constitutes about one-third the height 

 of the border; postero-lateral border converging at a variable angle 

 of 50 to 70, mature individuals being relatively narrower than 

 medium, and small-sized teeth, obliquity somewhat greater than 

 that of the opposite side, basal portion deep and rather deeply chan- 

 neled, the lower edge expanding posteriorly beyond the rounded, 

 inbeveled enamel fold, which envelopes the upper third of the height 

 of the border; inner margin making a slight sigmoidal curvature 

 from the obtuse anterior angle to the base of the coronal ridge 



