166 PALEONTOLOGY OF ILLINOIS. 



where it is suddenly deflected nearly at right angles to its longer 

 anterior course, gaining the posterior extremity in a slightly concave 

 course and nearly at right angles to the postero-lateral border. 

 Coronal region traversed by a prominent, sharply rounded ridge cul- 

 minating a little more than one-third the distance from the pos- 

 terior border, to which the moderately concave slope abruptly de- 

 scends, margined by the very narrow alation, the broad anterior 

 slope more gradually declining with gentle transverse concavity in 

 the vicinity of the antero-lateral border. Surface more or less dis- 

 tinctly undulated transversely, the dense enamel layer toward the 

 inner margin as also the worn area marked by impressed, irregu- 

 larly-spaced lines of growth parallel with the inner margin ; puncta- 

 tion fine and close. A small perfect tooth measures across the 

 inner margin 11 mm. ; length of antero-lateral border to inrolled 

 outer margin 7 mm. 



The above description refers to a form of peculiarly massive 

 strongly involute teeth of which there are several well-preserved 

 examples in the collections before us. In regard to the specific rela- 

 tions of these teeth with other known forms, the facts offer little 

 beyond conjecture. So far as relates to the superficial coronal 

 features they offer no appreciable contrast witL Deltodus spatulatus, 

 N. and W. ; but the value of these resemblances it is impossible at 

 the present time to determine. In the present instance we have no 

 evidence such as, in the case of Cochliodus, enables the grouping 

 together of divers dental forms with, to say the least, strong pre- 

 sumptive evidence in support of the interpretations relating to their 

 probable arrangement upon the jaws. On the contrary, with respect 

 to the detached remains of Deltodus and Sandalodus, although 

 unquestionably partaking of true cochliodont characteristics, the 

 student must be guided by their general similitude supplemented by 

 critical familiarity with the external structural features which two or 

 more species possesses in common, the persistence of which may 

 be inferred from what is known to occur in Cochliodus and imme- 

 diately allied genera; in order to detect even the possible specific 

 relations of the forms met with. Viewed in this light the associa- 

 tion of the present form with the supposed mandibular posterior 

 teeth of Deltodus spatulatus seems highly improbable. The strong 

 spiral inrollment and the comparatively narrow, rounded enamel 

 fold of the articular border, are not in consonance with the corres- 

 ponding features of the latter form ; hence it does not displace the 

 inferred relations of the median form ascribed to that species, 



