170 PALAEONTOLOGY OF ILLINOIS. 



separated by a low median ridge. Surface marked by more or less 

 distinct lines of growth parallel with the inner margin, and coarse, 

 irregular and irregularly elongate or confluent punctae. A medium - 

 size tooth measures across the inner margin 5 mm. ; greatest length 

 along the obligue border to point of inrollment, 11 mm. ; vertical 

 depth at middle, 4-5 mm. 



The foregoing diagnosis presents the normal condition of a pecu- 

 liar form of teeth which is represented by half a dozen, more or 

 less, perfect examples in the collections from the Upper Burlington 

 fish-bed. In two of these examples the inner half of the crown 

 shows a simple shallow depressed area denned by the slightly 

 raised border folds, while a third specimen shows throughout a 

 simple transverse convexity of the coronal surface. The latter 

 variety also possesses a narrower basal support, and in both f its 

 peculiarities it agrees with a solitary example in Mr. Springer's col- 

 lection from the upper or beds of passage fish-bed of the same form- 

 ation. The latter horizon has also afforded Mr. Springer a second 

 specimen which is undistinguishable from the variety described 

 above. 



Only two or three examples of this form have thus far been de- 

 tected in the Keokuk limestone, above which it has not been recog- 

 nized. The latter teeth are very like the typical specimens described 

 above, showing a similarly double-furrowed coronal surface, the only 

 marked distinction consisting in the greater size and more robust 

 figure of the Keokuk teeth. 



In consequence of the meagre data relating to this form it is dif- 

 ficult to determine the character of the variations alluded to 

 whether they are of specific or merely varietal importance. In the 

 accompanying description of the figures illustrating the several vari- 

 eties the following provisional terms are employed to distinguish 

 them : Deltodopsis? bialveatus, DJ convexus, DJ Keokuk. 



In the attempt to assign the present form to its proper generic 

 relations we are met at the outset by the difficulties incident to the 

 meagerness of the material for the purposes of comparative exam- 

 ination. The further characterization of the form discloses a slight 

 spiral inrollment, the direction of which indicates that the oblique 

 border is apparently homologous with the postero-lateral border of 

 typical Cochliodonts, and with this character is also associated the 

 obliquity of the inner margin from the posterior angle to the obtuse 

 anterior angle, and the somewhat marked flaring of the inferior 

 basal rim on the side corresponding to the antero-lateral border; 



