760 CAMBRIAN BRACHIOPODA. 



Cardinal area well defined. It is marked by transverse striae of growth parallel to the 

 base. The plane of the area extends backward over the cardinal line at an angle of 10° to the 

 plane of the margin of the shell. The delthyrium is well defined and covered by a convex 

 deltidium, the extent of which, however, is tmknown. The cardinal area of the dorsal valve 

 is short. No details of the structure have been observed. 



The cast of the interior of the ventral valve indicates short hinge teeth supported by, dental 

 plates. The markings left on the shell by the vascular system, as shown in casts, are two 

 maia vascular trunks wliich extend nearly in a straight line from the side of the cast of the 

 deltidial cavity obhquely outward to the anterior fourth of the valve, where they bifurcate. 

 The inner branch extends in toward the median line, while the lateral branches extend out- 

 ward, appearing to unite with the peripheral canal. The latter arches about the space that 

 was probably occupied by the ovarian areas, between itself and the main vascular trunks. 

 The spaces for the attachment of the muscles between the main vascular trunks appear to 

 have been large. No subdivision indicating the points of attachment of the different muscles 

 has been detected. At the posterior portion of the central area there was a narrow, elevated 

 space that posteriorly passes into the deltidial cavity. The pedicle muscles were probably 

 attached to the front part of this elevated space. Short, radiating striss near the margin 

 indicate the presence of minute vascular canals. 



In the interior of the dorsal valve the interior of the deltidial cavity supports a smaU, 

 well -developed cardinal process or callosity. The crura are short and small, with rounded 

 dental sockets beside them. Nothing is definitely shown of the vascular trunks or mesial 

 impressions in the dorsal valve. 



Ohservations. — In form this species is not unlike the convex forms that have been referred 

 to Billingsella coloradoensis (Shumard) from Trempealeau, Wisconsin. It differs from the 

 latter in having strongly marked surface plications and a narrower deltidium. From B. striata 

 it differs in surface characters ; also in having a more convex dorsal valve. 



Formation and locality. — Tipper Cambrian: (150a, 153, and 154) Limestones on Dry Creek, just above the 

 mouth of Pass Creek, about 5 miles (8 km.) north of Hillsdale; (151a) limestone in point overlooking Chum Canyon 

 on the west side of the Bridger Range ; (157 and 158) « limestone north of East Gallatin River, near Hillsdale; and (156b) 

 limestone north of East Gallatin River, at the north end of the Gallatin Valley; all in the Threeforks quadrangle (U. S. 

 Geol. Survey), Gallatin County, Montana. 



(152) Limestone on ridge between Chum and Cottonwood canyons; and (151) limestone in point overlooking 

 Chum Canyon; both on the west side of the Bridger Range, Gallatin County, Montana. 



Middle Cambrian: (91x) Limestone on Beaver Creek, 5 miles (8 km.) north of York, about 8 miles (12.8 km.) 

 north of Canon Ferry, Big Belt Mountains, Fort Logan quadrangle (U. S. Geol. Survey), Meagher County, Montana. 



(4j) Limestone at the head of Deep Creek, Canyon quadrangle (U. S. Geol. Survey), Yellowstone National Park, 

 Wyoming. 



Billingsella pumpellti Walcott. 

 Plate XCVII, figures 8, 8a-c. 



Billingsella purr>.pellyi Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 242. (Described and discussed as below 



as a new species.) 



General outUne subsemicircular, greatest width at the lunge hne or a little in advance 

 of it; considerable variation exists in the relative proportions of length and width; a ventral 

 valve 8 mm. long has a width of 9 mm.; the dorsal valve is more transverse, length 5.5 mm., 

 width 8 mm. The ventral valve is strongly convex, with the umbo arching over to the apex, 

 which is a little above the plane of the margins of the valve ; a very shght median fold occurs 

 near the front margin; dorsal valve shghtly convex and with a shallow median sinus. 



The surface is marked by low, sharp ribs with wide interspaces on which fine radiating 

 striae occur. Concentric lines and striae of growth cross the radiating striae and ribs. 



The characters of the cardinal areas of the valves are almost unknown; that of the ventral 

 valve appears to have been of moderate height and inclined but a little from the plane of the 

 valve; one interior of the dorsal valve shows a low cardinal process. The cast of the interior 



a 168 is the type locality. 



