268 NORTH AMERICAN INDEX FOSSILS. 



207. S. propinqua Hall. Middle Devonic. 

 Brachial valve somewhat more gibbous than in 5. multistriata, 



and pedicle beak and area generally somewhat more arcuate. 

 Vascular impressions on interior of valves bifurcating from one to 

 three times before reaching front of shell. 

 Onondaga of New York, Ohio, etc. 



208. S. tulliensis Hall. Upper Devonic. 

 Differs from 5. propinqua and S. multistriata in being more 



gibbous, with the beak of the brachial valve more elevated and 

 arching, and in having the divisions of the vascular impressions 

 running nearly parallel to each other or slightly converging, without 

 bifurcation. 



Tully of New York, and equivalent horizons of Nevada, etc. 



209. S. striatula (Schlotheim). (Fig. 321, d-f.) 



Middle and Upper Devonic. 



Brachial valve sinuate in front. Pedicle valve with a broad, un- 

 defined sinus, sometimes incurving the margin of the shell. Sur- 

 face very finely and evenly striated. 



Widely distributed throughout North America. 



210. S. macfarlani (Meek). Middle and Upper Devonic. 

 Very gibbous. Length in adults slightly greater than the 



breadth which is about 1 inch. Cardinal and umbonal regions 

 narrow. Hinge line scarcely equalling half the greatest width of 

 the shell. Pedicle valve depressed convex, bearing a broad sinus 

 anteriorly ; cardinal area of moderate height. Brachial valve very 

 gibbous with a greatly incurved umbo. About ten fine, bifurcat- 

 ing, radiating striae may be counted in the space of y 1 ^ inch. 

 New York, Iowa, Nevada, Northwest Territory. 



211. S. tioga Hall. (Fig. 321, £•-//.) Devonic. 

 Differs from 5. striatula in its smaller size, usually more pro- 

 nounced elevation of the pedicle valve and in the angular, fascicu- 

 late striae. 



Portage and Chemung of New York, Ohio. 



212. S. swallovi Hall. (Fig. 323.) Mississippi. 

 Large. Hinge line less than the width of the shell below. 



Pedicle valve depressed convex near the beak and flat at sides with 

 a broad depression extending from the middle to the front. 

 Brachial valve gibbous. Surface covered with fine and close 

 radiating striae and sharp concentric growth lines. 

 Burlington of Illinois, Iowa and Missouri. 



