320 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



loiiiths. It occurs in laro'o nunil)er.s and i,s a very striking- object, with 

 its dark, smooth shell in the burt'-o'ray sandstone. 



Forniafioii and Ayrr///Vy. —Upper Cambrian. St. Croix sandstone, 

 Taylors Falls, Minnesota. 



QUEBECIA, ne^w genus. 



The description of the species inchides that of the genus as there is 

 only one species now known. 

 Type. — Oholdla dree Bi 1 1 i ng-s. 



QUEBECIA CIRCE Billings. 



()},<ilill(i circc P>ii,T>ix(;s, Canadian Xat., ii. f^er., VI, 1872, p. 219; Am. Jour. 

 Sci., .'id Her., Ill, 1<S72, p. ;!57. — AValcott, Bull. V. S. (ieol. Survey No. 

 30, IKSH, p. lis, pi. x, iig. .Sa; Tenth Ann. Rej)!. IT. S. Geol. Survey, 1891, 

 p. (il 1, ])1. i.x.xi, iig. 8a. 



General form broad ovate, with fi'ont and sides uniforndy rounded; 

 planoconvex. Surface marked l)y tine concentric strii\? of growth. 

 A ventral valve 8.5 umi. in length has a width of 4.5 mm. A dorsal 

 valve 4.5 mm. in length has the same width; a larger dorsal valve, 

 11 lum. in length, has a width of 10.5 mm. Shell rather thick, and 

 composed of calcite in its present condition. 



The ventral valve has an elevated umbo terminating in a sharp, 

 upward pointing apex that may be perforate; from the umbo the sur- 

 face, in young shells, slopes so rapidly that the central and outer por- 

 tions of the shell are nearly Hat, l)ut in older shells moderately convex; 

 area short and divided midway l)y a relatively broad delthyrium that 

 is nearly covered by a convex deltidium; the inner surface is marked 

 by tine, elevated, rather widely separated, radiating strite and very 

 hue puncta^; a pair of very broad main vascular sinuses arch forward 

 from a point beneath the umbo and inclose a small, slightly elevated 

 visceral area; a strong cardinal tul)ercle occurs on each side of the 

 pseudo area, which was probably the point of attachment of the 

 posterior muscle, i. e., transmedian and anterior laterals. 



Dorsal valve gently convex in young shells, l)ecoming strongly con- 

 vex in old shells; no traces of an area have been ol)served — if present 

 it nmst have been short and very low, as the beak is nearly at the 

 plane of the margins of the shell; casts of the interior show that the 

 valve was thickened beneath the umbo by a ridge that separated what 

 may be called the umbonal cavity from the main cavity of the valve. 

 On the cast of an old shell a rounded, narrow, longitudinal, median 

 ridge divides the innl)onal cavity; on each side of the cavity and beyond 

 it there is the strongly marked path of advance of the transmedian 

 nniscle scars; in front of the transverse ridge two strong ridges that 

 extend toward the center of the valve occupy the position of the main 

 vascidar trurdvs; in one specimen a narrow groove extends from the 

 central nuiscle scar along the inside margin of the ridge. 



