SPIRIPER 337 



first gently convex, but as it approaches the margins it becomes more 

 curved and drops to the margin rather abruptly ; mesial sinus originating 

 at the beak where it is a slight, but well-defined groove, becoming less 

 well-defined anteriorly, growing rapidly broader and deeper towards the 

 front where it is very deep and subangular in the bottom, and is pro- 

 duced in a pointed lingual extension; the beak small, pointed, rather 

 strongly incurved ; the cardinal area of moderate height, the nearly flat or 

 gently concave inferior portion sloping posteriorly at an angle of about 

 130 degrees to the plane of the valve, the higher portion more concave 

 beneath the beak, the lateral margins sharply defined, sloping from the 

 beak to the cardinal extremities in nearly straight lines most of the dis- 

 tance, becoming convexly curved distally, the delthyrium of moderate 

 size, broader than long. Surface of the valve marked by rounded, bifur- 

 cating plications which occupy both the lateral slopes and the sinus, about 

 16 or 18 originate along the cardinal margin each side of the beak, al>out 

 the first ten of which bifurcate before reaching the middle of the valve, 

 so that from 26 to 28 are present upon each lateral slope at the margin of 

 the valve, in the sinus a median plication originates in the umbonal region 

 and continues to the front margin with one bifurcation near the middle 

 of the valve, the lateral plications of the sinus originate from the inner 

 margins of the bounding plications or from the bifurcation of such 

 plications, from 12 to 15 being present at the front margin. 



Brachial valve more convex than the pedicle, the greatest depth at the 

 anterior margin, the surface curving rather abruptly to the cardinal mar- 

 gin on each side of the beak, curving more gently to the antero-lateral 

 margins, compressed towards the cardinal extremities; the mesial fold 

 defined to the beak, scarcely elevated above the general surface in the 

 umbonal region, but becoming very greatly elevated in front where it 

 becomes subcarinate along the median line, its lateral surfaces curving 

 into the lateral slopes of the valve with only slight differentiation; beak 

 small, incurved. Surface marked by bifurcating plications entirely 

 similar in form and number to those of the opposite valve. 



The surface of both valves marked by fine, regular, sublamellose, con- 

 centric markings, about three of which occupy the space of 1 mm., and 

 by more conspicuous concentric lines of growth which become more fre- 

 quent and more or less crowded anteriorly. 



Remarks. This species is a peculiar one and may be easily recognized 

 by the extension of the convex fullness of the pedicle valve well out 

 towards the cardinal extremities, in association with a compression of the 

 brachial valve in the same direction. The strong elevation of the sub- 

 carinate fold of the brachial valve is another notable character of the 

 species, as is also the character of the bifurcating plications of the shell. 

 The species is perhaps most closely allied to.S. incertus, which it resembles 



