VEETEBRATES. 215 



tudinal depression is discernible a little within and parallel with the 

 outer border. The surface is smooth and full, the superficial struc- 

 tural details exactly agreeing with those observed in the preceding 

 form. In the unworn rounded outer border of one or two specimens 

 the coronal enamel presents faint rugosities apparently conforming 

 to the lines of growth, and which must have produced an exceed- 

 ingly delicate sculptured appearance in the surface. A small indi- 

 vidual measures in transverse diameter across the posterior margin 

 13 millimetres ; length along the inner border 14 mm. A medium- 

 size specimen is 35 mm., and 26 mm. in the same dimensions; and 



the largest individual as yet observed shows a lateral breadth of 53 

 mm. 



The above described forms undoubtedly pertained to one and the 

 same species. They occur in the collections in about equal numbers, 

 arid present precisely the same superficial features in all respects 

 save outline and contour. The supposed maxillary teeth apparently 

 vary but little in proportions. The larger teeth differing from the 

 small ones chiefly in the greater basal development in the region of 

 the outer border. A fragment of a very large tooth obtained by Mr. 

 Alexander Butters, representing an individual which was probably 

 not less than 90 millimeters in lateral diameter, presents the same 

 coronal contour in every particular, save the rounded posterior angle 

 of the inner border and the irregular, deeply scolloped posterior 

 margin, which may be likened to. a line of overhanging cliffs. The 

 latter specimen is also of interest on account of showing the perma- 

 nence of depth maintained by the enamel belt, which is dispropor- 

 tionately shallow as compared with the relative dimensions of the 

 tooth, while the basal portion is excessively developed. 



In regard to the supposed rnandibular teeth, there is observable 

 a much greater latitude in variability, especially in the proportions, 

 with age. The earlier formed teeth are relatively longer, while the 

 larger individuals show a greater transverse breadth, with which 

 latter character is also associated that noted in large examples of 

 the opposed form, in which the basal portion is developed into a 

 prominent flange along the exterior border. The collections contain 

 two large teeth, which differ so greatly from the normal condition 

 of medium and small-sized examples, that they might be mistaken 

 for representatives of a distinct species. The latter teeth are very 

 broad compared to the length, and to further obscure their proba- 

 ble actual specific identity, the posterior margin is in both cases 

 broken away, its outline conforming to the backward-arched lines of 



