342 PALAEONTOLOGY OF ILLINOIS. 



crest with fewer denticulations. We have also observed in the collection 

 of Mr. Van Hornb a very minute tooth, apparently referable to the 

 present form, in which the convex or posterior crown face is proportion- 

 ately much deeper, regularly arched laterally, the lateral extremities of 

 the basal fold more considerably curved upward in rounding the extrem- 

 ities, while the anterior face is exactly similar to that in the teeth 

 noticed. 



Notwithstanding their diminutive size, the specimens are usually 

 absolutely perfect, even the most delicate carina? and serrations in the 

 crest and upper edge of the coronal fold are distinctly and sharply 

 defined. Compared with other forms, the peculiar ornamentation of the 

 coronal region and the comparatively strong basal protuberance readily 

 distinguishes the present teeth from D. tumidus of the same horizon, as 

 also from D. mlnusculus of the Keokuk limestone. 



Position and locality : Not rare in the upper beds of the St. Louis 

 limestone ; Alton, Illinois, and St. Louis, Missouri. 



Desmiodus ? ligonifomis, St. J. and W. 



PI. Xa, Tig. 12-14. 



Teeth very small, subtriangular in general outline. Crown strong 

 and thick, gradually compressed towards the slightly obtuse crest, which 

 is nearly horizontal or but faintly acuminate and minutely denticulate, 

 each denticle being marked before and behind by a short carina, the 

 apical point bearing a few faint radiating carinas and which are rather 

 more widely spaced than their lateral diameter, lateral extremities 

 abruptly truncated ; concave face nearly plane above the coronal belt, 

 which latter occupies the broad inbeveled space, broadly arched down- 

 ward along the lower margin and rapidly contracted laterally, upper 

 margin less strongly arched and somewhat irregular in its direction, or 

 slightly arched upward in the middle, apparently without imbrications 

 but bearing a few obscure vertical plicae in the region of its greatest 

 vertical expansion ; convex face about two-thirds the hight of that just 

 described, lateral portions above the coronal belt nearly plane, median 

 region occupied by a strong angular vertical fold, which descends from 

 the apex and rapidly increases iu prominence towards the edge of the 

 coronal fold, from which point it gradually decreases in elevation, termi- 

 nating in a small basal protuberance, basal belt similar to that in the 

 opposite face, though somewhat less in depth, simple, inbeveled, mode- 

 rately arched upward in the middle and apparently considerably nar- 

 rowed in crossing the median ridge and nearly parallel with the crest. 

 Coronal surfaces, in certain conditions, ornamented with short irregular 

 vertical or slightly diverging thread-like striae, but usually, perhaps in 



