NO. 1120. PROCEEDINGS OF TEE NATIONAL MUSEUM. 715 



YORKIA WANNERI, new species. 

 (Plate LX, figs. 1, \a-\e.) 



Shell subcircular to suborbicular in outline, moderately convex, with 

 the apices of the valves marginal. Ventral (pedicle) valve highest 

 at the beak, which is truncated by a circular foraminal opening; car- 

 dinal slope angular and slightly incurved, so as to form a narrow 

 false area on each side of the slightly convex, rather broad pseudo- 

 deltidium. Xnmerous casts show the pseudodeltidium, false area, and 

 a large filling or cast of the foramen which extended obliquely backward 

 through the thickened nmbonal portion of the shell to the apex. On a 

 cast of the interior of the valve there are two elongated muscular or 

 vascular (probably the latter) impressions that extend from the antero- 

 lateral base of the foraminal opening inward to nearly the median line, 

 and then diagonally outward toward the anterior lateral margin of the 

 shell. There is also a slight median longitudinal ridge that corresponds 

 to a de])ression in the interior of the shell. (Plate LX, figs. l/>, Ic) In 

 other casts strije or lines only are shown radiating outward from the 

 base of the foramen. Fine transverse strife cross the narrow area and 

 then incurve and cross the pseudodeltidium. 



The dorsal (brachial) valve has a well defined area, with an obscure 

 pseudodeltidium parting it midway. The cast of the interior of the 

 valve shows two broad, shear shaped diverging ridges that extend from 

 near the apex to the center of the shell. Xumerous vascular markings 

 extend outward from the ridges. These ridges may indicate the mus- 

 cular sears or merely the main trunks of the vascular depressions. 

 The surface of the area is marked l)y fine transverse stri;e that abruptly' 

 incurve toward the front of tlie area, so as to follow along its anterior 

 margin to the pseudodeltidium. 



The surfaces of both valves, as shown in the casts, are smooth, except 

 where marked by concentric lines of growth. 



Shell substance unknown, but probably calcareous. It is dissolved 

 away in all the specimens in the collection, only the impression of the 

 shell remaining in the decomi)()sed arenaceous limestone. 



This species occurs in abundance, associated with BilUngseUa fes 

 tinata, Hyolithes americanus, tSalterella coiiica, new species, and frag- 

 ments of Olenellus. 



Formation and loadity. — Lower Cambrian, York Terrane, on left 

 bank of Codorus Creek, one-eighth of a mile below Meyer's JVlill, near 

 Emigsville, 4 miles north of York, Pennsylvania. (No. 2G433, U.S.X.M.) 



YORKIA? WASHINGTONENSIS, new species. 



(Plate LX, fig. 3.) 



Ventral (pedicle) valve longitudinally ovate, depressed, convex. 

 Area and pseudodeltidium invisible, but, from the character of the for- 

 amen and its resemblance to the foramen of l'. wanneri, it is probable 



