64 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Surface marked by sharp, fine, concentric striae and fine imbri- 

 cating lines of growth : on some shells low, irregular, more or less 

 obscure and interrupted radiating ridges occur. Shell of medium 

 thickness and built up of several layers or lamelk'e. The average 

 diameter of the valves is 3 mm. 



The interior of the ventral valve shows a short, flat area divided 

 midway by a narrow pedicle furrow ; the visceral area, which is 

 about one-third the length of the valve, is shown only in outline ; the 

 main vascular sinuses are strong and situated about midway between 

 the median line and the lateral margins of the valve ; the surface 

 outside the visceral area in both valves is marked by fine concentric 

 furrows and large scattered punctae, much like those of Obohis 

 (IVestoiiia) cscasoiii (Matthew) [1901, p. 270]. The interior of 

 the dorsal A^alve has a short area with a broad pedicle groove ; strong, 

 curved main vascular sinuses extend from beneath the area well 

 toward the front of the valve ; they are subparallel to the margin 

 and are situated about one-third the distance from the margin to the 

 median line of the valve ; the visceral area is outlined in about one- 

 half the length of the valve ; a narrow deep sinus extends from each 

 side of the anterior end and then curves outward to the front margin. 



Observations. — This shell was at first thought to be the young 

 of Oboliis tctoiiciisis ^^'alcott [1901, p. 684], but with the finding of 

 a good series it was found to have a nearly circular ventral valve 

 instead of subacuminate, as in O. tctoiiciisis, and it is less convex in 

 the same character of matrix. In form Obohis zcortliciii resembles 

 Obolus discoidcns (Hall and Whitfield) [1877, p. 205], but it dift'ers 

 in being more circular in outline and in having a thinner shell. 



Formation and Locality. — Upper Cambrian : Limestone of the 

 St. Charles formation [Walcott, 1908a, p. 6] about 250 feet (76.2 m. ! 

 above the Middle Cambrian, on the north side of Two Mile Canyon, 

 near its mouth, 2 miles (3.22 km.) southeast of Alalad, Oneida 

 County, Idaho. 



FORDINIA, new subgenus of OBOLUS 



This subgenus of Obohis is proposed for species having a Lingn- 

 Iclla-Wke outline and form with the development of a tendency to 

 form a platform or thickening in the valves in connection with the 

 attachment of the muscles in the ventral valve, and a thickening in 

 the posterior portion of the dorsal valve back of the central muscle 

 scars. The type of the subgenus, O. (P.) per feet us (see p. 65). has 

 these characters well developed. Another species, O. (F.) beUii- 

 iiis (Walcott) [19050, p. 2\^2,], has the cardinal area of the ventral 

 valve more united with the visceral area than it is in 0. {F.) per- 



