200 PALEONTOLOGY OP ILLINOIS. 



of the two contiguous teeth of the opposite parallel row, thus pro- 

 ducing an asymmetrical alternating arrangement only less symmet- 

 rical than7obtains in the modern Myliobates. That their real affini- 

 ties are with the Myliodonts there can be no doubt; but whether 

 they should be regarded as entitled to a specific family rank distinct 

 from their living allies, we do not deem ourselves prepared at the 

 present moment to affirm. As far back as 1862, Professor Agassiz, 

 as is well known to his students who were at that date and subse- 

 quently matriculated at Cambridge, had recognized the ordinal rela- 

 tionship of the Psammodi, and besides the typical Psammodus he 

 also included the several allied genera of the Aetobates type antici- 

 pated thus early in the earth's history by the genera Copodus, Pleuro- 

 gomphus, Pinnacodus, Labodus, Dimyleus, etc., which were recognized 

 from mountain limestone species occurring in the British Islands. 



Notwithstanding the incompleteness of our materials in certain 

 important particulars, we deem it something more than mere con- 

 jecture the assignment of the forms to definite positions upon the 

 jaws. As is well known, the maxillary dentaries of Myliobates present 

 a gently longitudinally arched condition so far as relates to the grind- 

 ing surface of the series of teeth with which the jaw was paved 

 across ; and that the mandible presents in comparison a triturating 

 surface strongly rolled in the same direction. The same state of 

 things may readily be appreciated in a suite of examples of Psam- 

 modus : The nearly plane or longitudinally slightly arched and trans- 

 versely more or less concave coronal contour of the teeth, may with 

 reason be regarded as having belonged to the upper jaw; and those 

 teeth which exhibit a contour almost the reverse of that just noticed, 

 being perceptibly more strongly arched longitudinally, also trans- 

 versely convex, with a more rapid convergence of lines projected 

 vertically to the coronal surface downward from either margin, indi- 

 cate for a series of teeth a more strongly rolled surface area than 

 in the above mentioned form, and corresponding in this essential to 

 the teeth composing the series of the mandible in the Myliodonts. 

 But the collections, both European and American, afford examples 

 of a form which, possessing essentially the outline and contour of 

 the previously mentioned forms, differ from them in their oblong 

 shape, being relatively longer than wide, and the depressed median 

 region and rounded condition of the coronal folds along the lateral 

 borders. All of the examples of the latter form which have come to 

 our notice are further distinguished by the relatively great depth of 

 the basal portion, which, however, in other respects intimately agrees 



