SPIRIFER 309 



sharply defined, subparallel with the cardinal margin through nearly the 

 entire length of the area, the lateral extremities of the area abruptly 

 truncate, its surface both longitudinally and vertically striate; lateral 

 slopes of the valve convex towards the middle, often with an obscure 

 oblique sinuosity extending from the side of the umbonal region to just 

 in front of the cardinal extremities, each marked by about 20 rounded 

 plications, the first two or three of which adjacent to the sinus usually 

 bifurcate in passing to the anterior margin, the other plications remain- 

 ing simple ; mesial sinus originating at the beak where it is sharply de- 

 fined, becoming less sharply defined anteriorly, it is shallow and rounded 

 in the bottom, a median plication originates near the beak and passes 

 without division to the anterior margin, on each side there are usually 

 two lateral plications originating from the inner margins of the bounding 

 plications, all the plications of the sinus being similar in form and size to 

 those of the lateral slopes. 



Brachial valve slightly less convex than the pedicle, with a narrow, 

 linear cardinal area, the greatest convexity of the valve near or posterior 

 to the middle ; mesial fold originating at the beak where it is bounded by 

 furrows deeper than those between the other plications of the shell, not 

 at all or but little elevated above the general surface of the shell in the 

 umbonal region, and but slightly elevated anteriorly, it is rounded in con- 

 tour and is marked by plications similar in form and number to those of 

 the mesial sinus of the other valve, all of which originate through the 

 division of a single plication at the beak ; lateral slopes moderately con- 

 vex antero-posteriorly, the curvature more abrupt towards the cardinal 

 margin, the surface somewhat compressed towards the cardinal extremi- 

 ties, marked by plications similar in form and number to those of the 

 opposite valve. 



The minute surface markings consist of fine concentric stride which are 

 crowded at intervals to form more or less conspicuous, irregularly ar- 

 ranged lines of growth which are often crowded towards the anterior 

 margin. 



Remarks. This species resembles several other Kinderhook members 

 of the genus Spirifer, but it may always be distinguished by the form of 

 the cardinal area of the pedicle valve, the two margins of the area being 

 essentially parallel almost to their very extremities, giving to the area 

 the form of a long, narrow parallelogram, while in the allied species the 

 lateral margins of the area slope laterally towards the hinge-line, giving 

 to the area a broadly triangular outline. From 8. vernonensis, the species 

 may also be distinguished by its less convex valves and less prominent 

 fold and sinus. 



The manner of growth which produces the characteristic form of car- 

 dinal area of this species is peculiar. In the younger growth stages of the 

 shell the hinge-line is much extended, into mucronate points, so that when 



