300 SYSTEMATIC PALEONTOLOGY 



Description. The above name has been employed for an unusual type 

 of operculum supposed to belong to some spiral shell like Madurea. The 

 particular gastropod possessing such a thick closure to the shell is un- 

 known. Possibly the shell was of such a nature that it was easily de- 

 stroyed, but the opercula occur often in considerable numbers. Several 

 distinct types of these opercula are known, but each holds its own par- 

 ticular form and marks a definite stratigraphic horizon. The one to which 

 the name Ceratopea keithi has been given, marks a zone in the Middle 

 Beekmantown throughout the Appalachian Valley, and as specimens are 

 usually common, this species is regarded as such an exceptionally valuable 

 guide fossil that the name Ceratopea zone has been applied to the strata 

 containing it. The various aspects of the species are illustrated in enough 

 detail on pi. XXXVI to make its identification certain. 



Occurrence. BEEKMANTOWN LIMESTONE (Ceratopea zone). Several 

 localities northeast and southwest of Halfway, Maryland, afford silicified 

 specimens of this fossil. A common and characteristic fossil of the Middle 

 Beekmantown in the Appalachian Valley from Pennsylvania to Alabama. 



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



Genus HELIGOTOMA Salter 



HELICOTOMA PLANULATOIDES Ulrich 



Plate XLII, Figs. 13-15 



Helicotoma planulatoides Ulrich, 1897, Geol. Minnesota, Pal., vol. iii, pt. 2, 

 p. 1034, pi. Ixxiv, figs. 28-30. 



Description. This species is closely related to the widespread Black 

 River species Helicotoma planulata Salter, but differs in that it has only 

 about four whorls instead of five and each descends slightly below the 

 level of the preceding. The umbilicus is somewhat narrower than in 

 H. planulata and revolving lines are totally absent. Specimens range in 

 width from 15 to 25 mm. 



Occurrence. CHAMBERSBURG LIMESTONE (Tetradium cellulosum 

 bed). Fort Loudon, Franklin County, Pennsylvania. Lowville limestone 

 of Kentucky and Tennessee. 



Collection. U. S. National Museum. 



