262 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvm. 



Nothino- is known of the intciior of the valves beyond an imperfect 

 cast of the area, of a dorsal valve. Ortliis /i(niil>fi)'gh<ni<l!< Walcott" is 

 a closel}' rehited species and it may be identical. 



Formation and locality. — Upper Cambi-ian to Lower Ordovician. 

 The Upper C-amln-ian locality is on Wolf Creek, 15 miles west-south- 

 west of Sheridan, Wyoming-, on the eastern slope of the Big- Horn 

 Mountains. 



The type specimens are from near the base of the Ordovician rocks 

 at Glen Eyre, canyon of Camp Creek, near Colorado City. The species 

 has also been found on west side of Front Creek, below Beyer Park, 

 Colorado. 



PLECTORTHIS DESMOPLEURA NYMPHA, new variety. 



This variety is based on sti'onj^l}^ convex ventral valves in which the 

 surface is marked by tine, radiating striae, separated into bands of 3 to 

 6 striae by stronger and more elevated stria?; concentric lines and often 

 ridges of growth occur that on some shells give an imbricated appear- 

 ance to the shell beyond the umbo. 



Formation and locality. — In passage beds between the Upper Cam- 

 brian and Ordovician, on north side of Tepee Creek, Big Horn 

 Mountains, Wyoming. 



PLECTORTHIS DIABLO, new species. 



This species is founded on a small, transverse shell that is marked 

 by a few prominent radiating cost;© that extend from the umbo to the 

 margin of the shell. In the cast the costa? have a sharp smimiit and 

 slope evenly to a narrow space between them. 



A ventral valve 9 mm. in height has a width of 14 mm., which is 

 about the average proportion of the specimens in the collection. The 

 casts of the ventral valve show a rather low area divided by a delthy- 

 rium, the inner angles of which have a rather strong tooth on each 

 side. The dental plates extend backward farther than in most species 

 of the genus, becoming attached to the bottom of the valve only near 

 the teeth, and not extending forward as in P. remnicha to form a 

 pseudospondylium. None of the specimens in the half dozen shells in 

 the collection show the muscular or vascular markings. There may 

 or may not be a mesial depression on either valve. 



Formation and locality.- — Upper Cambrian. St. Croix sandstone of 

 Trempealeau, Wisconsin. 



PLECTORTHIS DORIS, new species. 



Of this species a ventral and dorsal valve occur in the collection. 

 The general form is rounded subquadrangular. The ventral valve is 

 strongly convex with the greatest height at the umbo, from which the 



«Mong. VIII, U. 8. Oeol. Surv., 1884, p. 73, pi ii, figs. 5, 5a. 



