240 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



line rapi<ily slopinjj from the apex to the extremities, which are slightly rounded. 

 Hinge line straight, as long as the greatest width of the shell. Cardinal area broad 

 and high, divided in the middle by a triangular foramen, which is about as high as 

 wide. Surface of the venti'al valve nioileiately convex, marked by very fine radiat- 

 ing stri;c and also by several concentric lines of growth. Filling of the rostral cavity 

 and foramen large and prominent. Specimen, a cast in shale, of the ventral valve 

 only. 



()h,<<('rn(itl<))i.s. — The type specimen as descri])ed above is flattened in 

 the shale, and also appears to be compressed laterally. Uncompressed 

 specimens referred to this species from the siliceous limestones east of 

 Swanton show the ventral valve to have been rather stron^^ly convex, 

 and the dorsal valve moderately so. The outer surface as shown in 

 the cast, indicates that it was somewhat like that of B. color adoensis., 

 but that it differs in having tiner and more numerous thread-like 

 costas. A few traces of concentric stria? and lines of growth are pre- 

 served. The material is very unsatisfactory, but it is sufficient to 

 indicate that the genus occurs in association with Olenellus thompsoni 

 in the upper beds of the Lower Cambrian section. 



Foruiation and locality. — Lower Cambrian. Georgia terrane. 

 Silico-argillaceous shales: Parker's quarr}^, town of Georgia; and in 

 a gray siliceous limestone lentile, 2 miles east of Swanton, Vermont. 



BILLINGSELLA PLICATELLA, new species. 



Shell irregularly sulxjuadrate in outline. In the ventral valve the 

 cardinal line slopes toward the beak at an angle of from .5° to 10°, 

 while in the dorsal valve it is usually about 1° to 3°. The greatest 

 width of the shell is about the middle. There is some variation in the 

 relative proportions of length and breadth. Usually, however, the 

 ventral valve is fully as high as wide, while in the dorsal valve it is 

 slightly transverse. The ventral valve is rather strongly convex, the 

 dorsal valve being less so. A low, broad, mesial sinus occurs on nearly 

 all specimens of the dorsal valve, and sometimes a distinct sinus on the 

 ventral valve. 



The surface is marked by costai that gives it a plicated appearance 

 in many specimens; also concentric lines of growth and very fine, 

 slightly undulating strife. On some shells the costae and surface 

 markings have been almost entirely removed by abrasion, while in 

 others they are clear and distinct. There is quite a wide range of 

 variation in the strength and form of the surface markings. 



When the shell is exfoliated, fine, radiating lines occur toward the 

 front margins. The average size of the shell gives a width of about 

 8 mm., and in the ventral valve a nearly equal height. One specimen 

 has a width of 11 mm. 



Cardinal area well defined. It is marked by transverse stria? 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 



