VEETEB RATES. 275 



narrow intervening channel, and defined from the crown by a broader, 

 shallow depression; surface roughened by irregular sulci and minute 

 pits. Crown defined from base in front by a distinct channel, median 

 cone more or less deeply excavated at base in front, more or less irregu- 

 larly tapering to the apex, more compressed in front than behind, with 

 sharp cutting edges; lateral costae ascending to the apex in front, lim- 

 iting a rather broad furrow extending along the lateral margin, inter- 

 mediate space occupied by a few strong, sharp costae, which coalesce 

 with the lateral ridges at more or less irregular intervals ; posterior 

 face marked by a few simple costae somewhat more slender thau those 

 in front, and successively disappearing in the lateral margins ascend- 

 ing; lateral denticles normally four on either extremity, the exterior 

 pair long, but proportionately more slender than the median cone, more 

 or less divergent and recurved, with sharp cutting edges, and bearing 

 in frout two to four costae, which are sometimes bifurcated descending; 

 intermediate denticles alternating in size, the middle pair about half 

 the size of the exterior pair, their bases being continued in a marked 

 prominence into the antero-iuferior bosses of the base, ornamented with 

 one or two sharp costae, and flanked on either side by a smaller delicate 

 denticle, which usually bears a single sharp ridge in front. 



The description is based upon miuute teeth not uucommon in the St. 

 Louis limestone at Alton and St. Louis, and which present considerable 

 individual variation from the normal condition noticed above. In some 

 specimens there are only three lateral denticles on one side, and the 

 disposition of the coronal costae also presents less or more slight diver, 

 gence from that stated above, as shown in the figures of the two minute 

 teeth from the above mentioned localities. From the same formation 

 at Pella, Iowa, we have examples of apparently the same form, but 

 which attain nearly twice the size of the largest specimens yet observed 

 at the more southern localities, and which are otherwise distinguishable 

 by the absence, or rudimentary condition, of the smaller pairs of lateral 

 denticles, possibly the result of attrition or wear, and the apparent 

 though not relatively more numerous coronal costae in frout. 



This form, at first glance, appears to be closely allied to G. zygopm, 

 IS. and W., of the Chester formation; it is, however, readily distin- 

 guishable from that form by the greater number of lateral denticles 

 and sparser coronal costae, as well as its generally smaller size. In the 

 number and irregular development of the lateral denticles only does it 

 bear intimate comparison with G. inter costatus, of the Upper Burlington 

 limestone. 



Geological position and localities : Not uncommon in the upper beds 

 of the St. Louis limestone; Alton, Illinois, and St. Louis, Missouri; a 

 few specimens have been obtained from the calcareous shales in the 

 same formation near Pella, Iowa. 



