VERTEBRATES. 309 



Orodus parvulus, St. J. and W. 

 pi. vi, Mg. e. 



The form for which we have proposed the above designation is rep- 

 resented in our collections by two minute teeth, the perfect preservation 

 of which, however, readily enable the determination of their affinities 

 and distinctive peculiarities. The form may be distinguished from the 

 species so numerously represented in the same horizon, 0. plicatus, N. 

 and TV, by the marked dissimilarity in the outline and contour of tbe 

 crown, which is thinner, more evenly crested; it is gently arched between 

 the extremities, which latter are very slightly curved outward, slightly 

 constricted basally, culminating in the middle in a well-denned, though 

 not prominent median cone, the lateral wings, which are abruptly ter- 

 minated at their extremities, are occupied by numerous, closely set, 

 vertical or laterally deflected, bifurcated ridges, which terminate in del- 

 icate tuberculations along the sharp crest. The base is nearly or quite 

 equal to the lateral diameter of the crown, thin, deep, exceeding in 

 depth the elevation of the crown, obliquely produced behind and broadly 

 arched in both directions, anterior face broadly channeled and defined 

 above by the well-marked superior shoulder, inferior surface narrow, 

 beveled to the obtuse posterior border in nearly the same horizontal 

 plane as the crown, postero-lateral angles sharply rounded, anterior and 

 posterior faces dense, or delicately roughened or pitted. Lateral diame- 

 ter of tooth .27 inch, hight about .08. 



The above form also bears a close resemblance to 0. minutus, ST. and 

 W., of the Keokuk limestone; but the vertical ridges are stronger and 

 less elaborately wrought, besides there is no indication of the presence 

 of a delicately sculptured basal belt, such as occurs in the latter 

 form. From 0. carinatus, also from the Keokuk, it differs in the rela- 

 tively thicker crown, and more closely approximated, rounded vertical 

 ridges. As in 0. elegantulus, N. and W., of the Upper Burlington lime- 

 stone, the vertical plica} terminate in the crest along its lateral exten- 

 sions in minute tubercles, and the median prominence is also similarly 

 marked by radiating ridges, but the latter are much stronger and fewer 

 in uumber than those encircling the median cone in the latter species, 

 while the crest is much more compressed antero-posteriorly, and the 

 base proportionately much deeper. 



Position and locality: Rare in the upper beds of the St. Louis lime- 

 stone : Alton, Illinois. 



