ART. XXVIII -PALEONTOLOGICAL PAPERS NO. 6 : DESCRIPTIONS 

 OF NEW SPECIES OF INVERTEBRATE FOSSILS FROM THE 

 LARAMIE GROUP. 



By C. a. White, M. D. 



The fossils described in this paper were collected by the writer (unless 

 otherwise stated in connection with the description) from the strata of 

 the Laramie Group, during the season of 1877, in Colorado, Wyoming, 

 and Utah. Many other associated species were also collected, but only 

 the hitherto undescribed forms are noticed in this paper. 



Of the numerous invertebrate forms hitherto collected from the strata 

 of this great group, except some insect remains (to be described by Mr. 

 S. H. Scudder), and a few unstudied Ostracoid Crustaceans, all are mol- 

 luscau. 



CONCHIFERA. 



Genus YOLSELLA Scopoli. 



Subgenus Brachydontes Swainson. 

 Volsella (Brachydontes) regularis (n. sp.). 



Shell arcuate-subovate in marginal outline ; valves moderately con- 

 vex ; upper margin more or less strongly arched from beak to rear ; 

 thence with a continuous but stronger curve to the postero-basal mar- 

 gin, which is somewhat abruptly rounded to the gently concave base ; 

 front moderately narrow, slightly projecting beyond the beaks, and 

 abruptly rounded to the base; beaks depressed, scarcely perceptible as 

 such, and nearly but not quite terminal; hinge-margin short, nearly 

 straight; umbonal slope somewhat prominent, but conspicuous only by 

 increasing the apparent concavity of the basal part of the shell. Sur- 

 face marked by numerous, rather coarse, radiating lines, or small costse, 

 which increase in size toward the free margins of the shell. These 

 costse have generally a somewhat crenulated aspect, due in part to small 

 sinuosities in their course, and in part to being frequently crossed by 

 lines and undulations of growth ; denticles or crenulations of the short 

 front margin distinct. 



Length of the type-specimen 36 millimeters ; breadth at the widest 

 part 18 millimeters ; but several less perfect examples obtained at dif- 

 ferent localities indicate a much larger size, the largest of which must 

 have had a length of 6J centimeters. 



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