2 PKOCEEDIlSrGS OP THE NATION" AL MUSEUM vol. 7T 



symmetrically and regularly coiled, equally spaced, and equally 

 sized whorls. The outline of the tube in cross section is elliptical. 

 No demarcation is observed in cross section to indicate a peripheral 

 wall distinct from a central core. The material composing the tube 

 appears to be the same as that of the surrounding matrix. A petro- 

 graphic analysis - of the material of the tube shows that it consists 

 of fine-grained sand cemented with calcium carbonate and that the 

 dominant minerals present are calcite, quartz, feldspar, and biotite. 



Xenohelixf clarki apparently differs from Xenohelix marylandica 

 Mansfield in lacking, in cross section, two distinct parts — the pe- 

 ripheral wall and the central core. The species is named in honor of 

 Dr. Bruce L. Clark. 



Dimensions.— Holotype (Cat. No. 371106, U.S.N.M.) ; Length, 74 

 millimeters; diameter, 42 millimeters; maximum diameter of tube, 

 18 millimeters. 



The coils of another specimen (pi. 2, figs. 1, 2) obtained at the 

 same locality are more drawn out than those of the holotype. 



Occurrence. — Pine Canyon, Contra Costa County, Calif. Doctor 

 Clark informs me that both specimens were collected near the top 

 of the Monterey group at a horizon which he considers equivalent to 

 the " Temblor," in the middle part of the State, and are associated 

 with a marine middle Miocene fauna which includes such species as 

 Bruclarkia stanfordensis (Arnold), Area devincta Conrad, Macoina^ 

 new species, Pecten cf . perrin.i Arnold, and several other species. 



XENOHELIX? MEXICANA. new species 



Plate 1, Figures 2, 3 



The specimen selected as the holotype is incomplete, the whorls at 

 either extremity having been broken away. The specimen consists of 

 three symmetrically and regularly coiled, equally spaced, and equally 

 sized Vi^horls. The outline of the tube in cross section is elliptical, 

 being broadly angled at the periphery. Obscure spiral raised lines 

 occur near the periphery of the whorls. The material composing 

 the tube consists of ferruginous fine-grained sand and small mica 

 flakes and the surrounding matrix consists of dark-colored shale con- 

 taining particles of carbonaceous matter. 



Dimensions.— Holotype (Cat. No. 371107, U.S.N.M.) : Length, 

 36 millimeters; diameter, 25 millimeters; maximum diameter of 

 tube, 11 millimeters. 



Occurrence. — The label accompanying the specimens gives the 

 following information : " Black shales probably from the base of the 

 Exogyra costata horizon [Cretaceous]. Lower beds on top of Sierra 

 de la Azuf rosa on boundary line of Haciendas Mesillas and Azuf rosa, 



^ Analysis made by C. S. Koss, of the section of petrology, U. S. Geological Survey. 



