VERTEBBATES. 115 



external enamel layer is densest. A mature specimen measures in 

 greatest breadth between the inner angles 30 mm. more or less, 

 length along antero-lateral border 14 mm. 



With the exception of the narrow median teeth, the forms whose 

 specific identity is here recognized are represented in about equal 

 numbers of ii dividual teeth in the collections. The mandibular pos- 

 terior form exhibits some variation, especially in certain examples 

 which show faint revolving ridges or furrows over the broad anterior 

 slope of the coronal prominence, as illustrated in one of the figures 

 representing a tooth referred to this species. The latter feature also 

 recurs in some of the Streblodoid teeth of the upper jaw, as already 

 noticed, and although the absence of these obscure furrows and 

 ridges does not in all cases appear to be attributable to the acci- 

 dents of wear, their presence in certain individuals is not associated 

 with other characters which might be deemed sufficient grounds for 

 their separation from the specific relations here recognized. Amongst 

 the latter there occurs a single individual from Buffington creek, 

 which, in the relative proportions of the coronal regions with which 

 are associated other features, offer in the main rather marked con- 

 trasts with the superficial characteristics noticeable in the typical 

 examples of the form described above. These consist in the rela- 

 tively wide posterior lobe, the less distinct angular culmination of 

 its crest and gentler anterior declivity, proportionately narrower 

 median depression, more regularly transversely arched anterior lobe, 

 and the upward and forward deflection of the imbrications or trans- 

 verse rugae in crossing the median depression, constituting striking 

 features in contradistinction of those recognized in consequence of 

 their prevalence as normal in the typical representatives. Illustra- 

 tions of the latter specimen are also introduced, although its iden- 

 tity with the present species is only provisionally inferred. Also 

 amongst the few representatives of the mandibular median form the 

 normal condition described above is departed from in appreciable 

 degree, presenting a form distinguished by a relatively elevated reg- 

 ularly arched coronal prominence and correspondingly depressed 

 anterior slope, in which the oblique transverse rugae are more inter- 

 rupted in continuity than occurs in the case of the typical individ- 

 uals. The somewhat pronounced variations noted may indeed con- 

 stitute permanent characteristics such as a larger suite of specimens 

 might prove to possess specific value. But at the present time, 

 owing chiefly to the imperfect state of preservation of the material 

 in which many important details are masked, the evidence is not 



