GASTROPODA— TROCHOTURBINIDJE. 667 



Medium-sized, with small umbilicus and regularly sloping under 

 sides. 



Trenton (Galena) of Minnesota, Wisconsin, and Iowa. Similar 

 though larger specimens occur in the limestones of Lower Trenton 

 age in the Big Horn and Black Hill regions. 



Family Trochoturbinid;£ Koken. 

 LXI. Omphalotrochus Meek. 

 Large and ponderous euomphalids, with high spire, whorls 

 touching slightly, aperture oblique, with upper lip projecting. 

 Shoulder, and sometimes also the periphery, flattened or concave. 

 Mississippic-Carbonic. 

 194. 0. springvalensis White. (Fig. 923.) Mississippi. 



Fig. 923. Omphalotrochus springvalensis. (After Keyes. ) 



Whorls rounded, except for a slight flattening of the shoulder ; 

 gradually enlarging; height 55 mm., basal diameter 70 mm. 

 Kinderhook and Burlington of Iowa and Missouri. 



LXII. Poleumita Clarke and Ruedemann. 

 Small, and with the round whorls merely touching, and strongly 

 marked by spirals. Siluric. 



195. P. scamnata CI. and R. (Fig. 924, b.) Siluric. 

 With numerous sharp spirals ; faint transverse markings. 

 Guelph of New York and Canada. Monroe of Michigan ? 



196. P. crenulata (Whiteaves). (Fig. 924, a) Siluric. 



