VERTEBKATES. 379 



exceeding the bight of the convex crown-face, with which it is nearly 

 vertical. A specimen of ordinary size and proportions measures in 

 lateral diameter .5S inch, anteroposterior diameter .16, bight of convex 

 crown-face about .06, depth of abrupt face of root .09 inch. 



Of the present form there are several representatives in the collec- 

 tions, and taken together they constitute a well-marked group, the 

 individual variations of which do not depart in an unusual degree from 

 the normal condition. Notwithstanding its marked peculiarities, unmis- 

 takable as they appear in the examples before us, we have hesitated in 

 separating these teeth specifically from the last preceding form, T. belli- 

 cinctus, on the one hand, and T. polymorphic on the other, from either 

 of which forms the chief distinguishing difference in the present one is 

 the absence of coronal folds. Tet, it is impossible to overlook certain 

 other characters apparently of a permanent nature, which in either of 

 the above forms are probably due to accidental causes, as instanced 

 by the wedge shaped condition of the root. Again, in individuals 

 which bear evident signs of attrition, the same effects are produced in 

 the wearing away of the inferior basal angle of the convex crown-face, 

 and sulcatiou of the basal area, as has been observed under similar 

 circumstances in the teeth of the above mentioned forms. It would 

 seem very improbable that these permanent distinctive features are 

 attributable to the immaturity of the teeth, since it is well known that 

 the tissues of the coronal portion of the teeth of sharks are the first to 

 arrive at maturity; while in the present teeth, which seem to bear 

 greatest resemblance to the forms with which they are above compared, 

 the resemblance is largely due to the abrasion of the mature basal por- 

 tions. Its resemblance to certain forms of Polyrhizodus is remotely 

 indicated by the general similarity of coronal contour, also the obscure 

 vertical sulci often observed iu the edge of the root. But it is impos- 

 sible, with the data at present in hands, to prove a specific identity with 

 any of the above mentioned forms. Possibly the acquisition of more 

 extensive material may show these differences to be of varietal import- 

 ance only, since we should expect to find considerable variableness in 

 the teeth from different parts of the jaws, also those features originating 

 in accidental causes, as for example the the exfoliation by attrition of 

 the greater or entire portion of the superficial layer of the crown, thus 

 exposing to unusual abrasion the less dense tissues of dentine which 

 compose the bulk of the crown and the entire basal region, producing 

 under like circumstances similar results, and giving rise to these pecu- 

 liarities of individual teeth the specific identity of which may be 

 exceedingly difficult, if not impossible, to ascertain. In the study of 

 the fossil teeth of this class, such difficulties are of constant recurrence, 

 and iu the present instances, the desire to comprehend the actual rela- 

 tionship, and not the recognition of a new form, has induced us to 



