VEKTEBKATES. 213 



PSAMMODUS PLENUS, St. J. and W. 



PI. XVI, Fig. 1-4. PL XVII, Fig. 1-4. 



Teeth attain large size. Form supposed to pertain to the upper 

 jaw subrhomboidal, or of a laterally elongate-trapezoidal outline, 

 gently arched antro-posteriorly. Margins nearly parallel, somewhat 

 irregularly undulated, making a shallow concavity and then very 

 slightly arched toward the antero-hiteral angle, the reverse occurring 

 in the posterior margin where the concavity is in the approach to 

 the postero-lateral angle, both margins inbeveled or perpendicular 

 to the crown surface, the anterior having greatest apparent obliquity, 

 and denned above by the narrow coronal belt which constitutes less 

 than one-fourth the depth of the face ; the inner border is nearly 

 straight, showing a faint angulation a little posterior of the middle, 

 and a narrower truncated articular facet at the posterior angle, the 

 coronal folds of the margins forming a continuous belt well denned 

 from the moderately channeled basal portion, which terminates in- 

 feriorly in a narrow rim ; the outer border is very gently arched, 

 converging at an angle of about 15, more or less, with the oppo- 

 site border, and generally sharply rounded at the anterior angle, 

 terminating in a more or less produced, acute, angle posteriorly ; in 

 small or earlier-formed teeth the outer border is sharply inbeveled 

 above and somewhat deeply .channeled, limited by the narrow flange 

 or basal rim below; but in larger individuals the basal portion in- 

 creases in prominence, forming a massive border extending consid- 

 erably beyond the coronal limits and terminating in a more or less 

 produced spur at the posterior angle. Coronal surface smooth, 

 gently arched longitudinally and between the lateral borders, making a 

 broad, shallow concavity, forming an angle along the inner border 

 and sharply rounded in the narrow coronal fold along the outer 

 border, where it is well defined from the basal portion in small and 

 large examples alike. The crown surface is uniformly minutely 

 punctate, the pores being considerably spaced and confined within 

 the easily discerned vertical prisms. However, the surface structure 

 varies according to the state of preservation. In much worn exam- 

 ples the punctae are coarse and crowded, while in others under an 

 ordinary lens the radi of calcigerous tubes around the orifices of 

 the pores and the minute prismatic structure of the inter-spaces are 

 beautifully revealed. Again, the lines of growth generally describe 

 a segment of a broad circle with the convexity toward the posterior 



