110 PALAEONTOLOGY OF ILLINOIS. 



The only specimen, representing this species in the collection, is somewhat 

 mutilated, but still exhibits well marked characters by which it may be readily 

 recognized wherever found. The rugosity of the surface is much like that of 

 J's. ruffosus, Ag., and it also resembles that species in its transverse wrinkles 

 or furrows; but, unless a portion only of the original thickness of the tooth is 

 represented in the specimen before us, it must have been far thinner and lighter 

 than any form of P. rugoms which has been described. 



Figures 5, 6«, represent the upper surface and a transverse section of the 

 tooth, of the size of nature. 



Formation and locality : Chester limestone, Monroe county, Illinois. 



PSAMMODUS? KHOMBOIDEUS, N. and W. 

 PI. XI, Figs. 6, 6 a. 



Teeth rhomboidal in outline, thick, strongly arched in both 

 directions ; triturating surface, sub-rhomboidal, with rounded 

 angles, forming a prominent disc or boss, surrounded by the 

 roughed and irregular base. On two sides the crown is some- 

 what compressed, inclosing a prominent angle or ridge which 

 rises to the summit of the tooth. Other portions of the tritu- 

 rating surface are arched in both directions. The base on two 

 sides is much depressed, on the third less so, and on the fourth 

 somewhat turned up. It is rough and porous; in the central 

 portion of the tooth, thick, but thinning toward the edges, 

 which are irregular. The base is below deeply excavated, 

 strongly concave in the line of its shortest diameter, nearly 

 straight in its longest. Most of the triturating surface is uni- 

 formly punctate, the pores in the center being relatively small 

 and remote. Near the margin they are larger and more con- 

 fluent, ultimately merging into the vermicular roughening of 

 the base. 



This tooth is considerably unlike those hitherto described, and as yet little 

 can be said in regard to its relations with them, or those with which it is asso- 

 ciated, in the locality where it is found. We have placed it provisionally in 

 the genus Psammodus because it seems most to resemble some of the varied 

 forms which still remain included in that genus. It is by no means certain, 



