VERTEBKATES. 107 



longitudinally, and the under surface shows the same peculiar 

 structure as the upper; length 2 inches; greatest breadth 8 

 lines. 



Figures 10, 10 a, 10 b, are top and side view and profile section, natural size; 

 figure 10 c, magnified view of the crown surface. 



Formation and locality: Hamilton group, Calhoun county, Illinois. 



Genus PSAMMODUS, Ag. 



Gen. Char. — " Teeth oblong, wide, thick, flat above, without 

 cones or keels; surface porous or minutely rugose; base of the 

 same form as the crown, very thick, coarse, osseous." 



PSAMMODUS POROSUS, Ag. ? 

 PI. XT, Figs. 1, la, 16. 



Teeth oblong or nearly square, the angles of one end being 

 right angles, of the other, one acute and one obtuse ; tritura- 

 ting surface more or less concave, in the line of its largest 

 diameter; slightly arched transversely; in perfect specimens 

 smooth and polished, every where dotted with fine and nearly 

 equal pores marking the termini of the calcigerous tubes; 

 thickness one half to one fourth of length; base of the same 

 form as the crown, coarse and cellular. 



The specimens in the collection which we have referred to this species, are 

 undistinguishable, by any well marked characters, from those described by Prof. 

 Agassiz. (Pom. Foss. Vol. 3, p. 112, Atlas, Vol. 3, tab. 13). 



Figures 1, la, lb, give the top, side and end views, natural size. 



Formation and locality: Chester limestone, Chester, Illinois. 



PSAMMODUS ANGULARIS, N. and W. 



PI. XI, Pig. 2, la, lb. 



Teeth small, thin, oblong, or trapezoidal in figure; surface of 

 crown plain or slightly curved, generally smooth except as 



