/ L 



228 PALEONTOLOGY OF ILLINOIS. 



of variable depth, which is due to abrasion while in use. The 

 coronal surface is enveloped in a thin layer of enamel, beneath 

 which as also in the worn areas the punctate structure is clearly 

 discernible, the punctae inclosed in vertical prisms, as also observed 

 in relation to the superficial structural features of Psammodus. 



The foregoing diagnosis applies to a form which, from the moder- 

 ate antero-posterior convexity of the crown, and which was com- 

 municated to the series of teeth, evidently pertains to the maxillary. 

 Associated with the above form, Mr. VanHorne has discovered a 

 fragment of a tooth, exhibiting precisely the same structural feat- 

 ures above noticed, which we are led to regard as the representa- 

 tive of the opposed form belonging to the lower jaw. 



Fi<j< * The latter or mandibular form, in a general 



way agreeing in outline with the maxillary teeth, 

 is distinguished by its relatively greater trans- 

 \ verse diameter as compared to the length, strongly 

 arched longitudinally, with a narrow depressed 

 belt parallel with the lateral borders of the 

 crown which form a slightly upraised fold, be- 

 yond which the basal border extends in a prom- 

 inent rim. The anterior and posterior walls of 

 the base, as also the inferior surface enveloped 

 in the dense layer protecting the coarser cellular 

 structure composing the greater portion of its 

 thickness, are precisely as observed in the above 

 described form of the upper jaw. 



The last noticed form is comparable to the 

 teeth to which Professor Agassiz gave the name 



Mandibular form. ....... 



Fig. a. Triturating sur- Labodus, of which two species are indicated, viz., 

 'Fig. i>. Transverse pro- L. prototupus, Agass., and L. planus, Agass., 



file from inner margin. .... 'f^^ i -r> L 



Fig. <>. Longitudinal from the mountain limestone of Ireland. But in 



profile. 



the present instance all the facts seem to point to generic identity 

 with the first described form, which is unmistakably congeneric with 

 the teeth designated by Professor Agassiz under the above generic 

 appellation. As to the homological relations of the two forms al- 

 luded to above, to briefly recapitulate : The slight longitudinal 

 convexity of the first described form, indicating for the series of 

 teeth associated in the same row a very moderately arched longi- 

 tudinal contour; and in the case of the latter form the strong con- 

 vexity of the crown from behind forwards, indicating a corresponding 

 strongly rolled outline for the series of two or more individuals, in 

 both instances constitute characters consonant with what actually 



