VERTEBRATES. 85 



from the plain anterior neck. Surface along inner margin often 

 preserving the external coating of dense opaque enamel, with more 

 or less distinct lines of growth; otherwise the surface exhibits a 

 minute pitted structure produced by the relatively widely-spaced 

 punctse. A mature tooth measures in greatest width between the 

 inner angles 10 m m. ; length of antero-lateral border to point of 

 inrollment 4.5 m m. 



Thus far only the posterior teeth of the upper jaw have been 

 identified. The fragmentary condition of the majority of the speci- 

 mens often renders their identification with one another a difficult 

 matter, not to mention the necessary comparisons in order to 

 determine their relationship with other similar forms occurring in 

 the same strata. In their general aspect these teeth bear a striking 

 resemblance to Deltoptychius expansus ; but they may be dis- 

 tinguished by the absence of the median sulcus and subordinate 

 ridge in front of the posterior coronal prominence, and specifically 

 by their narrow transverse diameters. 



Associated with the above mentioned teeth, certain forms repre- 

 senting the mandibular posterior teeth occur, which might be mis- 

 taken for more specimens of the homologous form of Deltoptychius 

 expansus. None of the latter specimens are in a state of preserva- 

 tion to show their distinctive features with sufficient clearness to 

 dispel all uncertainty in regard to their generic relations. Their 

 coronal region is divested of the superficial enamel coating, expos- 

 ing the minute close punctate structure such as appears in worn 

 surfaces, and presenting a nearly plain coronal prominence without 

 subordinate narrow belt along the antero-lateral border, as occurs 

 in typical examples of Deltoptychius ; in the latter respect these 

 teeth approach the corresponding form referred to Orthoplewodus, 

 and while we are in doubt as to their actual affinities, they are 

 provisionally placed in the present specific association, of which 

 characteristic examples of both forms are presented in the illustra- 

 tions. Compared with the homologous teeth of Deltoptychius expan- 

 sus, they are further distinguished by the less oblique backward 

 course of the inner margin, which forms nearly a right angle with 

 the antero-lateral border, the appreciably narrower and less up- 

 raised alation, which is marked near the border by a slight furrow 

 or angulation from which rises the narrow marginal belt along that 

 side, and which is inbeveled to the channeled basal portion. The 

 deep antero-lateral border is abruptly truncated, half its height 

 enveloped in the belt of coronal enamel, which is well defined from 



