. GASTEROPODS. 157 



the matrix forming the cast of the inside. Some species have 

 a thickened shell, with the whorls barely in contact, or even 

 separated toward the aperture. In instances of this kind the 

 internal casts have much the appearance of some of the forms 

 for which Sowerby established the genus Phanerotinus. But 

 with the latter have evidently been included a number of evo- 

 lute Straparolli. 



Straparollus valvataformis SHUMARD. 



Plate li, tig. 8. 



Straparollus valvataformis Shumard, 1863: Trans. St. Louis Act d. Sci. f 

 vol. II, p. 105. 



Shell small, closely resembling 8. spergenensis. 

 Horizon and localities. Cambrian limestone : Ozark 

 county. 



Straparollus obtusus (HALL). 



Plate li, fig. 5. 



Euomphalus obtusus Hail, 1858: Geology Iowa. vol. I, p. 523. 

 Straparollus obtusus Keyes, 1889: Proc. Acad. Nat. Sci., Phila., p. 296. 

 Straparollus obtusus Keyes, 1890: American Geologist, vol. V, p. 197, pi. 

 i, figs. 2a-c. 



Shell large, planorbiform, composed of five to six regu- 

 larly rounded volutions ; spire on a level with, or slightly be- 

 low, the upper surface of the last whorl ; suture very deeply 

 impressed ; upper surface of the volutions very slightly flattened 

 on the inner side near the suture ; umbilical region very broad 

 and shallow ; aperture circular. 



Horizon and localities. Lower Carboniferous, Lower 

 Burlington limestone: Louisiana (Pike county), Hannibal. 



This form was the first of the group recognized in the 

 Kinderhook rocks along the Mississippi river, and is one of 

 the most characteristic gasteropods of this horizon at Burling- 

 ton (Iowa), and elsewhere. At the latter place it occurs in 

 the oolitic layer a few feet below the Burlington limestone, and 

 is easily distinguished from all the congeneric species of the 

 locality by its large size often attaining a diametric measure- 

 ment of more than six centimeters its greatly depressed spire, 



