496 NORTH AMERICAN INDEX FOSSILS. 



1. Interior ribs 1 8 or more 396. P. (Amusium) mortoni. 



I. Interior without ribs 2. 



2. Ears subequal, diverging at a sharp angle above the beaks a. 



a. Height greater than breadth 393. P. (Entolium) operculiformis. 



a. Height and breadth subequal 392. P. (Entolium) aviculatus. 



2. Ears not diverging at a sharp angle above the beaks b. 



b. Surface crossed by concentric folds II. 



II. Height greater than length ....394. P. (Syncyclonema) rigidus. 

 11. Height and length nearly equal aa. 



aa. Folds extending across ears 368. P. conradi. 



aa. Folds not extending across ears. ..371. P. burlingtonensis. 



b. Surface not crossed by concentric folds 22. 



22. Surface smooth bb. 



bb. Ears subequal 369. P. simplicius. 



bb. Ears unequal 394. P. (Syncyclonema) rigidus. 



22. Surface radially striated cc. 



cc. Height greater than breadth f. 



f . Anterior ear narrow, separated by a deep byssal sinus 

 from body of shell. 



390. P. (Camptonectes) pertenuistriatus. 

 f . Anterior ear broad. Byssal sinus shallow. 



372. P. argillensis. 



cc. Height and breadth subequal ff. 



f f . Shell oblique to hinge, large. 



388. P. (Camptonecte%) bellistriatus. 

 ff. Shell nearly erect. ..389. P. (Camptonectes) extenuatus . 



B. Surface of valves with radial ribs II. 



II. Ribs few, about 6 370. P. quiuqucnarius. 



II. Ribs many 3. 



3. Ribs subequal c. 



c. Radially striate 2>?>- 



23- Striae rough or spiny dd. 



dd. Ribs flat-topped 383. P. (Chlamys) magnolia. 



dd. Ribs subangular 380. P. (Chlamys) perplanus. 



dd. Ribs round fff. 



f ff . About 6 fine striae on top of each rib. 



382. P. (Chlamys) jeffersonius. 

 f f f . About 3 coarse striae on top of each rib. 



381. P. (Chlamys) madisonius. 



33. Strise not spiny ee. 



ee. Ribs about 24 373. P. (Chlamys) stearnsii. 



ee. Ribs about 14 379. P. (Chlamys) ivahtubbeanus. 



c. Not radially striate 44- 



44. Ribs bifurcating 376. P. (Chlamys) choctavensis. 



44. Ribs simple .ff> 



ff. Ribs increasing by implantation at front of shell. 



378. P. (Chlamys) johnsoni. 



ff. Ribs not increasing by implantation 4f . 



4f. Ribs 25-30 377. P. (Chlamys) greggi. 



4f. Ribs 12-18 *■ 



*. Shell ^inch'high.. 375. P. (Chlamys) nebrascensis. 



