DESCRIPTION OF SPECIES BRACHIOPODA 303 



Description. — Shell of medium size, subsemicircular in outline, wider 

 than long, the greatest width along the hinge line, cardinal extremities 

 angular or a little rounded. Pedicle valve strongly convex, strongly 

 arched from beak to front, the median line often describing nearly a 

 semicircle; the beak rather small, pointed, and strongly incurved; car- 

 dinal area of moderate height, concave, its ventral margin sharply de- 

 fined, sloping laterally from each side of the beak to the hinge line at the 

 cardinal extremities, the slope becoming more abrupt as it approaches the 

 end of the hinge line; mesial sinus deep, subangular, originating at the 

 point of the beak and widening rapidly to the front, where its anterior 

 portion is often produced into a lingual extension; it is smooth at the 

 beak, but anteriorly it is marked by a single median plication and by 

 eight or ten others which originate through the successive bifurcation of 

 the lateral bounding plications; lateral slopes convex except along the 

 cardinal margin and toward the cardinal extremities, where they become 

 slightly concave. Brachial valve nearly as convex as the pedicle, most 

 elevated at the anterior extremity of the mesial fold ; beak small, strongly 

 incurved, cardinal area narrow, becoming attenuate toward the cardinal 

 extremities ; mesial fold depressed in the posterior half of the valve, fre- 

 quently becoming strongly elevated, subcarinate and sometimes slightly 

 recurved toward the anterior margin; it is smooth at the beak, but 

 marked anteriorly by eight or ten plications which originate through the 

 successive bifurcation of the initial median rib; lateral slopes convex 

 except toward the cardinal extremities, where they become somewhat 

 flattened. Lateral slopes of both valves marked by mostly simple, rarely 

 bifurcating, rounded plications, fifteen to twenty-two being present upon 

 each side of the fold and sinus, those toward the cardinal border becom- 

 ing successively smaller; surface also marked by fine, concentric, sub- 

 lamellose lines of growth which become more or less crowded toward the 

 front. 



The dimensions of a very perfect individual are : Length of pedicle 

 valve from umbo to front margin, 23 millimeters ; length of brachial 

 valve, 17.75 millimeters; width of shell, 31 millimeters; thickness, 19 

 millimeters. The dimensions of a large pedicle valve, in part restored, 

 are: Length, 32.5 millimeters; width, 52 millimeters; convexity, 11 

 millimeters. 



Remarks. — Swallow's type specimens of this species were collected at 

 the locality now known as Fern Glen. The smaller examples of the spe- 

 cies resemble S. marionensis, but they are much more convex in both 

 valves, with a much more elevated mesial fold and impressed sinus ante- 

 riorly. The shape of the cardinal area of the pedicle valve is also differ- 



