OBOLELLID^. 597 



of the valve is usually more or less tumid, a ridge of growth separating it from the anterior 

 portion of the shell; area nearly on the plane of the margins of the valve and divided midway 

 by a narrow, deep, pedicle furrow; casts of the interior show that the area formed a shelf on 

 each side of the pedicle furrow. Dorsal valve transversely ovate; a narrow, short area and 

 a slight median ridge are indicated on a cast of the interior. 



This shell is small. A large ventral valve measures, length, 7 mm. ; width, 6 mm. ; a dorsal 

 valve, length, 6 mm.; width, 6.5 mm. 



Oiservations. — The characters of tliis little shell, as far as they are known, are those of 

 Oholella. In my original description I gave an erroneous interpretation to the cast of the 

 interior of the ventral valve. At that time I did not know much of the interior of Obolella, 

 and also considered the cast of the space beneath the area beside the pedicle groove as indicating 

 the existence of plates as in Pentamerus. In the absence of the shell or fine casts it is impossi- 

 ble to determine conclusively whether a foraminal tube existed, as in Oholella atlantica Walcott, 

 or not; one cast indicates that such was the case. The form and long area distinguish this 

 from other species of the genus. 



Formation and locality. — lower Cambrian: (16f) Sandstone at the mouth of Little Antietam Creek, near 

 Eakles Mills; (47d)<i sandstones 1 niile (1.6 km.) east-southeast of Smithsburg; (47e) sandstone on Observatory Hill, 

 2 miles (3.2 km.) south of Keedysville; and (47f) sandstone at Eakles Mills, 2 miles (3.2 km.) south of Keedysville; 

 all in Washington County, Maryland. 



(32) Sandstone on the south slope of Stissing Mountain, Dutchess County, New Yorh. 



(49b) Sandstone 2 miles (3.2 km.) northwest of Emigsville, York County; (49c) sandstone in ridge just north of 

 the railway station in Emigsville, York County; (49d) sandstone 3 miles (4.8 km.) east of Waynesboro, Franklin 

 County; (49e) sandstone 1 mile (1.6 km.) south of Mount Zion Chm-ch, and 4 miles (6.4 km.) northeast of York, York 

 County; and (49f)'* sandstones of Mount Holly Gap, South Mountain, Cumberland Coimty; all in Pennsylvania. 



(47a) Sandstone on the southwest side of Sailings Mountain, 2 miles (3.2 km.) east of Natm'al Bridge, Rockbridge 

 County, Virginia. 



(47c) Sandstone 2 miles (3.2 km.) west of Harpers Ferry, Jefferson County, West Virginia. 



Obolella mobergi Walcott. 

 Plate LV, figures 3, 3a-f. 



? Obolus? sp. Kjerulf, 1873, Om Skuringsmserker, Glacialformationen, Terrasser, og Strandlinier, etc., 2: Sparagmit- 



fjeldet, p. S3, figs. 10 and 11. (No text reference.) 

 Obolella mobergi Walcott, 1901, Proc. U. S. Nat. Mus., vol. 23, pp. 673-674. (Described and discussed as below as a 



new species.) 



General form ovate, with the ventral valve subacuminate and the dorsal valve obtusely sub- 

 acuminate; valves moderately convex; beak of the ventral valve slightly elevated above the 

 plane of the shell; beak of the dorsal valve curved down to the plane of the margin. Surface 

 of the shell marked by concentric lines and striae of growth, and in some examples by rather 

 strong, radiating; broken, and slightly irregular raised lines of the same character as those of 

 Bicia gemma (Billings). The shell is foi-med of a thin outer layer, and numerous inner layers or 

 lamellse over the anterior half of the shell. These imbricating inner lamellffi are oblique to the 

 outer layer, very much as in Oholella (Glijptias) favosa (Linnarsson). 



A ventral valve 10 mm. in length has a wddth of 9 mm. A dorsal valve 9 mm. long has the 

 same width. 



The area of the ventral valve is of medium length and tipped back from the edge of the 

 valve to meet the beak; it extends well out onto the cardinal slopes, and is divided midway by 

 a narrow furrow. The surface of the area is marked by rather strong transverse striaj crossing 

 the area. On each side of the base of tlie pedicle tube there is a rather strong undercut which 

 in the cast is replaced by a marked toothhke projection. The area of the dorsal valve is narrow. 

 The cast of the pedicle tube or foramen is large, cylindroconical, and encircled by a number of 

 rather strong lines of growth (PI. LV, fig. 3c). 



The casts of the interior of the ventral valve show a central visceral area, very much like 

 that in 0. crassa (Hall); also two strongly marked vascular sinuses that extend well into the 



a Doubtfully identified from this locality. 



