VERTEBRATES. 189 



the length. Postero-lateral border nearly straight, with a slight 

 oblique course outward and forward, enamel fold narrow, basal por- 

 tion relatively deep, slightly channeled; antero-lateral border con- 

 verging toward point of inrollment at a moderate angle with the 

 opposite border, the relatively deep basal rim projecting beyond the 

 coronal fold ; inner margin gently arched from the rounded posterior 

 angle, with a slight sigmoidal curvature in passing to the subacute 

 anterior angle ; as usual the edge is inferiorly beveled. Crown sur- 

 face very gently and uniformly convex transversely, the enameled 

 inner portion showing transverse imbrications or lines of growth 

 parallel with the inner margin, with the usual delicate rugose 

 appearance, beneath which the tubular structure is discernible, and 

 which in worn surfaces assumes the minute, closely arranged punc- 

 tate character. Greatest transverse diameter across inner margin, 

 13 mm. ; length along postero-lateral border to point of inrollment, 

 about 8 mm. 



The form above described is represented by a solitary specimen, 

 in the collection of Mr. Van Home. The tooth is in good preserva- 

 tion, save that the outer margin has been entirely worn away, 

 presenting a thick, blunt emarginate edge, the inrolled extremity 

 on the posterior side being much more worn than at the opposite 

 side. The inferior surface of the tooth, however, plainly shows the 

 direction of the oblique inrollment, as indicated by the course of the 

 longitudinal striation. The form agrees in all essential particulars 

 with the homologous teeth noticed in connection with Sandalodus 

 complanatus, and S. l&vissimus ; at the same time it also agrees, in the 

 character of the surface punctation and the delicate rugose mark- 

 ings of the enamel layer, with the other forms of the species to 

 which we have referred it. However, in the supposed associate 

 forms we have failed to detect the presence of transverse lines of 

 growth, such as constitute a marked feature in the enamel layer of 

 the present form. The uniform transverse convexity of the coronal 

 region and its outline readily distinguish the present tooth from its 

 congeneric allies. 



Referring to the synonymy it will be observed that the form orig- 

 inally described under the name Deltodus rhomboideus, N. and W., 

 is transferred to the specific category embracing the form first 

 noticed under the term Sandalodus spatulatus, the two forms respec- 

 tively representing the supposed posterior teeth of the lower and 

 upper jaws of the same species. These forms most intimately agree 

 in the details of coronal structure, while they differ one from the 



