316 NORTH AMERICAN INDEX FOSSILS. 



i / . Sinus not produced anteriorly 339. S. arenosus, 



1. Shell of medium size and convexity b. 



b. Cardinal area narrow 2 7 . 



2' '. Shell granulose 365. S. marionensis. 



2 / . Shell notgranulose , L f . 



b f . Hinge line extended 364. S. centronatus. 



b' . Hinge line not extended 371. S. striatus. 



b. Cardinal area moderately high if. 



3 / . Hinge line shorter than width of shell below. 



373. 6". rockymontanus. 

 y. Hinge line forming greatest width of shell (usually 



recognized by internal mold) 361. S. disjunctus. 



\\ Plications on fold and sinus extending only part way to the beak and 

 hence differing from the other plications 2. 



2. Plications angular 369. S. leidyi. 



2. Plications rounded c. 



c. Cardinal extremities rounded 4/. 



4'. Plications of sinus unequal 342. S. grieri. 



4'. Plications of sinus equal 363. S. keokuk. 



\> '. Plications of sinus obscure 337. S. concinnus. 



c. Cardinal extremities angular 370. S. increbescens. 



* Plications covering all the surface but fold and sinus ff. 



|f Plications crossed by more or less conspicuous concentric lines, often pro- 

 ducing a lamellose appearance XXX- 



XXX- Shell small, usually less than 1 inch wide 3. 



3. Hinge line usually shorter than width of shell below d. 



d. Furrow in median fold 331. S. vanuxevii. 



d. No furrow in median fold 5 / . 



5 / . Plications obsolescent ^1- S. corallinensis. 



5 / . Plications well developed 330. S. crispus. 



3. Hinge line extended e. 



e. Plications 4-7 on each side fold and sinus... 329. S. sulcatus. 

 e. Plications many on each side fold and sinus 6 / . 



6 / . Front broadly rounded 344. S. varicosus. 



6 / . Front narrowly rounded owing to the often mucro- 



nate hinge extremities 362. S. subatteniuitns. 



XXX Shell 1 inch or more wide except if young 4. 



4. Furrow in fold of brachial valve, hinge line extended f. 



f. Cardinal areas narrow 7 / . 



7 / '. Plications few and absent from the cardinal extremities. 



357. S. s culpti lis. 

 7 / . Plications many 354. S. mucronatus. 



f. Cardinal areas wide &'. 



8 r . Beak prominent 356. S. conwbrinus. 



& / . Beak small 360. S. mesicostalis. 



4. No furrowin fold of brachial valve g. 



g. Hinge line shorter than width of shell below g / . 



9 / . Beak of pedicle valve much elevated. 



337. S. concinnus. 

 9 / . Beak of pedicle valve not much elevated. 



336. S. cyclopterus. 



