SPIRIFER 329 



Description. Shell of medium size, wider than long, the greatest width 

 along the hinge-line, the cardinal extremities angular. The dimensions of 

 a nearly complete specimen, one of the cotypes, are : length of pedicle 

 valve 27.5 mm., length of brachial valve 24.8 mm., width 41 mm., thickness 

 17.5 mm., height of cardinal area 5 mm. The dimensions of another one 

 of the cotypes, a pedicle valve, are : length 25 mm., greatest width 41 mm., 

 height of cardinal area 7 mm. 



Pedicle valve most convex posterior to the middle, the surface sloping 

 rather abruptly from the umbonal region to the cardinal margin, much 

 more gently to the antero-lateral margins, somewhat compressed towards 

 the cardinal extremities ; beak small, pointed and moderately incurved ; 

 cardinal area arched, becoming more curved towards the beak, its lateral 

 margins sharply defined and sloping gently from the beak to the cardinal 

 extremities, the surface marked by vertical striae; lateral slopes of the 

 valve each bearing 28 to 34 narrow, rounded plications, some of which 

 bifurcate; mesial sinus rounded in the bottom, originating at the beak 

 where it is sharply defined, becoming more or less ill-defined anteriorly, 

 it is marked by from 10 to 14 plications similar to those upon the lateral 

 slopes of the valve, the median plication originates near the beak and 

 continues to the front margin either as a simple rib or bifurcating, the 

 lateral plications originate from the inner margins of the two bounding 

 plications or through the bifurcation of plications so originating. 



Brachial valve less convex than the pedicle, its greatest convexity near 

 the middle ; mesial fold originating at the beak, it is narrow and scarcely 

 elevated in the umbonal region but becomes moderately elevated an- 

 teriorly where it is ill-defined and rounded or subangular in contour, 

 it is marked by from 10 to 14 plications similar to those of the opposite 

 valve which originate from a single plication at the beak; lateral slopes 

 moderately convex towards the middle of the valve, somewhat compressed 

 towards the cardinal extremities, marked by plications similar in form 

 and number to those of the opposite valve. 



The minute surface markings of the shell consist of fine, more or less 

 irregular, raised, concentric lines. Both valves are also marked by 

 numerous, imbricating, sublamellose lines of growth, which are crowded 

 near the front of most shells. 



Remarks. This species has frequently been identified as 8. subcequalis, 

 but a critical examination of many individuals, including the cotypes, has 

 shown that these two species are not synonymous. S. tenuicostatus has 

 somewhat finer and more angular plications, and also much more con- 

 spicuous sublamellose concentric markings. 



The species is sometimes associated with 8. lateralis, and in a crushed 

 and imperfect condition there may be some difficulty in distinguishing 

 between them, but S. lateralis has more acutely angular cardinal extremities 

 with a carinate median fold, and does not possess the conspicuous con- 



