CARBONIFEROUS PERIOD. 125 



Wasatch range ; pass between Rush and Cedar Valleys ; and east side of 

 Mount Nebo, Utah ; top of Grass Movintain, Ely range ; Fossil Hill ; 

 Camp Apache ; old Potosi mine ; Tenney's Ranch ; Kaibab Plateau ; and 

 at the confluence of White Mountain and Black' Rivers, Nevada. 



Genus ORTHIS Dalnian, 1828. 

 Orthis Pecosii Marcou. 



Plate IX, fig. 5 a, h, c, d, and e. 



Orthis Pecosii Marcou, 1858*, Geol. North America, 48. 



Orthis carhonaria Swallow, 1858, Traus. St. Louis Acad. Sci., i, 218. 



Orthis carbonaria Meek, 1872, U. S. Geol. Surv. Nebraska, 173. 



Shell small, sublenticular; outline subcireular or subovate ; length and 

 breadth nearly equal, but sometimes the length is a little the greatest ; front 

 margin regularly roiuided or slightly emarginate ; hinge-line very short, 

 less than half the breadth of the shell. Ventral valve having its greatest 

 convexity at the umbo, often flattened a little at the front, but without a 

 definite mesial sinus ; beak small, pointed, somewhat prominent, and arched 

 over the small, well-defined area, which arches with it. 



Dorsal valve more convex than the ventral in old shells, its greatest 

 convexity being behind the middle, generally having a mesial flattening 

 extending from the umbo to the front, but no definite mesial sinus ; area 

 distinct, but smaller than that of the other valve ; beak small, not prominent. 



Surface of both valves marked by fine, close-set, radiating striae, which 

 increase mainly by implantation, but occasionally by bifurcation ; the 

 striae crossed by fine concentric lines of growth, and, toward the front of 

 old specimens, by imbricating lines. 



The striae often show small pores upon their backs, apparently marking 

 the fonner places of minute tubular spines. 



* Orthis Pecosii, Retzia Mormonii, Ehijnchonella Vta, E. Eockymontana, aud Spirifer Eockijmontana were 

 pulilisbed by Marcou in his Geology of North America. I have obtained satisfactory evidence that the 

 work was published as early as March 1, 1858. Vol. xv of the Bulletin de la Society G^ologique de 

 France contains a statement that a copy of the book was sent to that society on April 20, 1858. In the 

 same year, Shumard and Swallow published a paper containing descriptions of the three first-uaraed 

 species, under othernames, in the Transactions of the St. Louis Academy of Sciences, but that ijublication 

 was not made until about the first of June. In December of the same year, Hall published in the Geo- 

 logical Eeport of Iowa, Spirifer Eockymoniana as S. ojnmus ; and in 1860, McChesney published E. Eocky- 

 montana as E. ctoniccfomiis. It thus appears clear that Marcou is entitled to priority of all five of the 

 names above given, as stated in the synonymy heading the descriptious of those species in this report. 



