312 . SYSTEMATIC PALEONTOLOGY 



The form of the volutions, the slit-band, and the striate, almost 

 lamellose, surface markings characterize this interesting shell which has 

 hitherto been known only from the Trenton rocks of the Ohio Valley. 



Occurrence. MARTINSBURG SHALE (Sinuites bed). Chambersburg, 

 Pennsylvania. Trenton at Covington, Kentucky. 



Collection. U. S. National Museum. 



Genus MICROGERAS Hall 

 MlCEOCEEAS INOENATUM (Hall) 



Plate L, Figs. 21, 22 



Microceras inornatus Hall, 1845, Amer. Jour. Sci. and Arts, vol. xlviii, 



p. 294. 

 Cyrtolites (Microceras) inornatus Meek, 1873, Geol. Surv. Ohio, Pal., vol. i, 



p. 147, pi. xiii, figs. 4a, b. 

 Cyrtolites subcompressus Meek, 1873, Geol. Surv. Ohio, Pal., vol. i, p. 147 



(under C. inornatus). 



Description. Shell minute, the largest diameter being 1.25 mm. or 

 less, consisting of two rapidly diminishing volutions which form a spire 

 equally depressed on either side and obtusely carinated or angular upon 

 the back. The carina is most conspicuous near the aperture and gradu- 

 ally becomes obsolete. The aperture is somewhat quadrangular and the 

 surface is smooth. 



This shell occurs almost invariably in association with several species 

 of Cyclora, similar minute or dwarfed gastropods of larger species such 

 as Cyclonema or Lophospina. In themselves these shells are of little value 

 as horizon markers and it is only in association with other species that 

 they are useful. 



Waagen supposed the species of Microceras to represent " embryonic 

 volutions of bellerophontes," but they are more likely dwarfed varieties 

 or embryqnic stages of some carinated genus such as Cyrtolites. 



Occurrence. MAETINSBURG SHALE (Sinuites bed). Chambersburg, 

 Pennsylvania. Mohawkian and Cincinnatian rocks of the Ohio Valley, 

 etc. 



Collections. Maryland Geological Survey, U. S. National Museum. 



