STRATIGRAPHIC TAXOKOMY 



665 



Mohawkian epoch in the Ordovician, the Niagaran in the Silurian, and 

 the Onondaga-Hamilton stage in the Devonian. 



The IlcUcotoma ? is one of three closely related species or varieties 

 of an undescribed type of gastropods^, related on one side to Helicotoma, 

 on the other to Liospira, but with sufficient characters of its own to 

 justify the erection of a new genus. The shell is low, conical or discoid 

 in shape, 20 to 40 mm. in diameter, broadly umbilicated, and narrow- 

 whorled. The upper surface of the whorls is marked with two rows of 

 nodes, one on the peripheral band, the other between this and the suture. 

 A similar row of nodes is found on the under side. One of the species 

 is very coimnon in the fauna just listed. The same variety is associated 

 with a larger species in one of the beds of the Yellville formation. The 

 latter is very common at many localities in southern Missouri, and being 

 easily recognized makes an excellent guide fossil. Both forms occur 

 also in central and western Texas. A third variety is found, sometimes 

 with either of the others, in the upper part of the Canadian limestone 

 (hitherto referred to the Knox) between Bristol and Wytheville, Vir- 

 ginia. It has been seen also in the Lehigh Valley in Pennsylvania. At 

 most of these localities these neat shells are associated with one or more 

 of the Ceratopea and the Bathyurus. 



The name Ceratopea has been used by me during the past five or six 

 years to designate a peculiar type of operculum supposed to belong to 

 some gastropod allied to Maclurea. The spiral shell itself is unknown, 

 but as the opercula are striking and often very abundant fossils, and 

 have great value as guide fossils, a distinctive name is desirable. Several 

 distinct forms of these opercula are known. Two of them were figured 

 many years ago by Billings in his Paleozoic Fossils, volume 1, page 243, 

 figures 228 and 230. At least five others are represented in the col- 

 lections in the U. S. National Museum. To make these fossils of greater 

 use in discussions and correlations of Canadian deposits, the generic 

 name Ceratopea is now formally proposed for them. As genotype, T 

 have selected the species figured by Bassler without description in Bulle- 

 tin N"o. IT-A. Virginia Geological Survey, 1909, plate 20, figure 3. 

 The specific name Tceithi is here applied to the genotype, Mr. Arthur 

 Keith, of the U. S. Geological Survey, having collected at Trundles 

 Crossroads east of Knoxville, Tennessee, some of the best specimens seen. 



It is a curved variety of Ceratopea heithi that occurs so abundantly 

 in the lower middle part of the Arbuckle limestone in the Wichita uplift 

 of southwestern Oklahoma. The same variety is also a common fossil 

 in central Texas. In both localities, as also in central Alabama, it is 

 associated with a fourth species of the genus difl'ering from C. l-eithi — 



XLIV — Bull. Geol. Soc. Am., Vol. 22, 1010. 



