GASTEROPODS. 189 



Horizon and localities. Lower Carboniferous, Burlington 

 limestone: Hannibal. 



This species appears to be genetically related to 0. acuti- 

 rostre ( Hall ), from the Keokuk ; and eventually the two may 

 prove identical. 



Orthonychiaformosum (KEYES). 



Plate liii, fig. 2. 



Platyceras f&rmosum Keyes, 1888: Proc. Am. Philosophical Soc., vol. XXV, 



p. 242, figs. 8-9. (Reprint, p. 14.) 



Capulus formosus Keyes, 1890: American Geologist, vol. VI, p. 9. 

 Capulus fonno&us Keyes, 1890: Proc. Acad. Nat. Sci., Phila., p. 1G4, pi. 



ii, fig. 8. 



Shell arcuate, enlarging rapidly to the ample, irregularly 

 pentalobate aperture ; longitudinally plicated. 



Horizon and localities. Lower Carboniferous, Kinderhook 

 group : Marshall county, Iowa : Burlington limestone : Louisi- 

 ana. 



The two original specimens are attached to the ventral 

 surfaces of specimens of Dorycrinus immaturus W. & Spr. At 

 Louisiana the form occurs in chert nodules. This species re- 

 sembles, in some respects, C. paralius ( W. & W.), but is simply 

 arcuate instead of being coiled. 



Orthonychia boonvillense (MILLKR). 



Platyceras boonvillense Miller, 1891 : Geol. Sur. Indiana, 17th Ann. Rep., 

 Adv. sheets, p. 82, pi. xiv, figs. 15-1. 



A rather small, regularly arched form, with a subcircular 

 aperture. 



Horizon and localities. Lower Carboniferous, Keokuk 

 limestone : Boonville. 



