VERTEBEATES. 109 



PSAMMODUS? SEMI-CYLINDRICUS, N. and "W. 

 PL XI, Fig. 4, 4 a. 



Teeth thick and strong, sub-semi-cylindrical in form, oblong 

 in outline, twice as long as wide; crown strongly arched 

 transversely, to a less degree longitudinally; enameled surface 

 occupying all the higher portions of the tooth; where unworn, 

 granular and shagreen-like; where worn, regularly porous; 

 around the margin the rugosity partaking of the vermicular 

 character of the surface of the base ; base transversely concave, 

 thinning laterally into sharp and irregular margins. 



In its form this species somewhat resembles Fs. canaliculatus, McCsfy, (Brit. 

 Pal, Foss., p. 643, pi Sg, Jig. 12.) but is at once distinguishable from it by the 

 absence of the plications and gibbosity of the crown, which form the marked 

 characters of that species. 



The shagreen-like rugosity of the triturating surface, to which allusion has 

 been made in the above specific description, is not peculiar to this tooth, but is 

 common to several species of this and allied genera. It is only shown where 

 the enamel coating is unworn, and disappears upon the exposed portions of 

 long used teeth. It is due to the presence of raised rings about the extremities 

 of the enameled tubes, and is doubtless a provision of nature to prevent the 

 slipping of the teeth over the smooth surfaces of shells or other hard substances 

 which it was their function to crush. In worn teeth the roughness which they 

 retain is due to the pores or pits formed by the sections of the enameled tubes. 



Figures 4, 4a, represent one of these teeth as seen from above and in profile. 



Formation and locality : Keokuk limestone, Warsaw, Illinois. 



PSAMMODUS RETICULATUS, N. and W. 



PL XI, Figs. 5, 5 a. 



Teeth oblong, more or less flattened, thin; crown irregularly 

 arched transversely, nearly straight longitudinally, slightly 

 depressed at the ends, showing a few remote transverse sulci; 

 rugosity of triturating surface nearly uniform, coarse, reticu- 

 lated by the intersection of short, tortuous lines and furrows, 

 running parallel with the longest diameter of the tooth. 



