SPIRIFER 315 



to the cardinal margin on each side of the beak, to the antero-lateral 

 margins the curvature is more gentle, an ill-defined oblique sinuosity 

 passes from the cardinal margin on each side of the beak to just in front 

 of the cardinal extremities, the valve sometimes a little compressed 

 towards the cardinal extremities ; beak rather small, pointed, incurved ; 

 cardinal area rather low, arched, the curvature becoming greater towards 

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

 cardinal extremities in a nearly straight line or with a gently convex 

 curve, surface of the area vertically striate ; lateral slopes convex 

 towards the mesial sinus, usually slightly compressed towards the cardinal 

 extremities, marked by from 13 to 15 simple, rounded plications which 

 grow successively smaller towards the cardinal extremities, the last two 

 or three sometimes being very faint or almost obsolete; mesial sinus 

 originating at the beak where it is sharply defined, becoming less sharply 

 defined anteriorly, it is rather shallow and rounded or subangular in 

 the bottom, marked by a median plication which originates near the beak 

 and passes without division to the anterior margin, on each side are from 

 1 to 3, but mostly 2, lateral plications which originate from the inner 

 margins of the lateral bounding plications. 



Brachial valve a little less convex than the pedicle, the greatest con- 

 vexity near or posterior to the middle ; mesial fold sharply defined to the 

 beak, the bounding furrows deeper and stronger than those between 

 the other plications, but little or not at all elevated in the posterior half 

 of the shell, becoming slightly elevated anteriorly ; it is marked anteriorly 

 by plications similar in form and number to those of the sinus of the 

 opposite valve, all of which originate through the division of a single 

 plication at the beak; lateral slopes convex towards the middle of the 

 valve, becoming compressed towards the cardinal extremities, marked by 

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



The minute surface markings are obscure in all of the specimens ex- 

 amined because of the condition of preservation, but concentric markings 

 of the shell are clearly indicated which are sometimes crowded into 

 more or less conspicuous lines of growth. 



Remarks. This species has sometimes been identified as 8. marionensis, 

 but it differs markedly from that species in the shape of the cardinal 

 area and also in the absence of bifurcating plications upon the lateral 

 slopes. From 8. vernonensis it differs in the less strongly convex valves, 

 in the shallower sinus, and in the absence of the conspicuous lingual 

 extension in front, and in the lower mesial fold, also in the mucronate 

 extensions of the hinge-line. These mucronate extensions are frequently 

 missing on the specimens as they occur in the form of more or less im- 

 perfect casts, but in every specimen in which this portion of the shell 

 is uninjured, they have been observed, and it is believed that the character 



