270 rROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvm. 



PLECTORTHIS REMNICHA TEXANA, new variety. 



The rounded, subquadrate form, and the arrangement of the radi- 

 ating costs© servew to differentiate this variet}'. This costal vary con- 

 siderably on different shells, but the tendency of the larger number 

 is toward a surface characterized by narrow, evenly spaced, radiating 

 ribs; tine radiating striae occur on the ribs and interspaces. A shell 

 that is doubtfully referred to this variety occurs in the silicious Middle 

 Cambrian limestones, 50 feet above the conglomerate series, St. Francis 

 County; also in section 22, township 35 of Iron County, Missouri. 



Forniation and locality. — Upper Cambrian. Cold Creek Canyon, 

 Burnett County; on Cold Creek, north end of Sponge Mountain, 2 

 miles south of San Saba County line in Llano county, Texas. Arbuckle 

 limestone, NE. \ sec. 2, T. 2. S., R. 1 E., Ardmore quadrangle, 

 Indian Territory. 



PLECTORTHIS REMNICHA WINFIELDENSIS, new variety. 



This variety is characterized by having tiner and more uniform 

 costte than any of the shells referred to /*. rcDinicJia. The costa? 

 increase by interpolation rather than Infurcation in the true sense of 

 the word. The shorter costa? begin as very narrow, sharp ridges, 

 merging into the sides of the larger costte below the summit of the 

 latter, or they ma}' arise entirely on the interspaces between the costje. 

 The pseudospondylium is less strongly developed than in P. remni- 

 cha. The young shells are usually narrower at the hinge line than the 

 adult; but this feature is sometimes present in large shells. 



Forniation and locality. — Upper Cambrian. St. Croix sandstone, 

 Winiield, Pilot Knol), and doubtfully Trempealeau, Wisconsin. 



PLECTORTHIS RETROFLEXA Matthew. 



Oonambomtes -plana var. retroflexa de Verxeuil, Beitrage zur Geognosie des 



Russischen Reiches, 1830, p. 77, pi. xxv, figs. 1, 2. 

 Clitambonites {Gonamhonites) plana var. retroflexa Matthew, Trans. Roy. Soc. 



Canada, 2d ser., I, 1896, p. 268, pi. ii, figs. la-c. 

 Clitambonites planuK ntroflexns Schuchekt, Bull. U. 8. (Teol. Survey No. 87, 



1897, p. 184. 

 Billingsella retroflexa iNIatthew, (ieol. .Sur. Canada, Rep. Cambrian Rocks, Cape 



Breton, 1903, p. 148, pi. x, ligs. 2a-e. 



This species is distinguished by the tine radiating costa? on the ven- 

 tral valve and coarser cost;© on the dorsal. The tine, longitudinal 

 strife are similar to those of P. re/nnicha. In a specimen of the ven- 

 tral valve, the pseud<)spond3dium, area, and base of the main vascular 

 sinuses are shown. Specimens of the dorsal valve show the cast of 

 the strong cardinal process, dental sockets, and crura, and the ovarian 

 areas and visceral area between. Doctor Matthew describes other 

 features of the interior of the valves. 



