SPIRIFER 367 



more or less distinctly in pairs to the anterior margin of the shell ; mesial 

 sinus subangular or somewhat rounded in the bottom, defined to the 

 beak, often produced anteriorly in a nasute or lingual extension, the 

 plications usually somewhat fainter than the larger ones on the lateral 

 slopes ; they consist of a median one which originates near the beak and 

 passes without division to the anterior margin ; on each side of the sinus 

 there are two or three lateral plications which are given off serially from 

 the inner margins of the bounding plications. 



Brachial valve equally or a little more convex than the pedicle, its 

 greatest convexity near or in front of the middle ; mesial fold sub-angular 

 or sometimes rounded, well defined to the beak, moderately or rather 

 highly elevated anteriorly, marked by a median furrow corresponding to 

 the median plication of the opposite valve, and by three or four lateral 

 plications, all of which join to form a single one at the beak; lateral 

 slopes each convex towards the mesial fold, becoming flattened or a 

 Httle concave towards the cardinal extremities, marked by plications 

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



The minute surface markings consist of fine concentric lines which are 

 somewhat irregular in the strength of their development, and by occa- 

 sional much stronger lines of growth. 



Internally the muscular impressions of the pedicle valve are broadly 

 ovate and deeply impressed, the shell in the umbonal and cardinal regions 

 being much thickened. 



Remarks. This species is especially characteristic of the Crawfords- 

 ville shale of Keokuk age. It is characterized by its elongate hinge-line 

 which in young individuals is distinctly acuminate, by the rather broad 

 area of the pedicle valve with subparallel margins, and by the rather 

 coarse, dividing plications. 



Horizon. Keokuk (Crawfordsville beds of Indiana). 



SPIRIFER MONTGOMERYENSIS n. sp. 

 Plate LV, Fig. 9 



Description. Shell above medium size, wider than long, the greatest 

 width along the hinge-line, the cardinal extremities angular, acuminate in 

 the type specimen. The dimensions of a pedicle valve are : length 37 

 mm., width 75 mm., convexity 15.5 mm., height of cardinal area 7 mm. 



Pedicle valve moderately convex, the greatest convexity posterior to 

 the middle, the umbonal region broad, the surface curving abruptly to 

 the cardinal margin for a short distance on each side of the beak, the 

 curve to the antero-lateral margins gentle, somewhat compressed toward 

 the cardinal extremities ; beak small, acuminate, moderately incurved ; 

 cardinal area gently arched, the curvature becoming stronger toward the 

 beak, its lateral margins sharply defined, sloping gently from the beak 



