486 NORTH AMERICAN INDEX FOSSILS. 



XCI. Aviculopecten McCoy. 

 Pectiniform, inequilateral, inequivalve, with right valve usually 

 less convex than the left. Hinge line straight with both anterior 

 and posterior ears. Surface usually radially sculptured. Liga- 

 ment internal, extending to both sides of the beak in many shallow 

 grooves roughly parallel to the long hinge line. Resiliifer present. 

 Teeth absent. Pallial line simple. Siluric-Carbonic. 



A. Large, — exceeding ii inches in diameter I. 



I . Striae fasciculate 343. A. providencesis. 



I. Striae spiny 335. A. scabridus. 



I. Striae simple and smooth 334. A. princeps. 



B. Of medium size, — about I inch in diameter II. 



II. Rays in sets of two 337. A. duplicatus. 



II. Rays spiny 354. Acanthopecten carboniferus. 



II. Rays in bundles of irregular size 333. A. fasciculatus. 



II. Rays 4-6, roughened by scales and separated by finer striae I. 



I. Ribs prominent only on lower half of shell 352. A. vanvleeti. 



1. Ribs prominent from near the beak to the margin 351. A. maccoyi. 



II. Rays of nearly but not quite equal size and crossed by crowded and regular 



concentric striae; hinge-line crenulated 364. Crenipecten winchelli. 



II. Rays of equal size but varying lengths 2. 



2. Beak acute and projecting above the hinge line 353. A. oklahomaensis. 



2. Beak nearly a right angle and scarcely projecting above the hinge line. ..a. 



a. Ears prominent 349. A. occidentalis. 



a. Ears small.. 345- A. curtocardinalis. 



II. Rays regularly alternating 3. 



3. Smaller rays only near front 348. A. weberensis. 



3. Smaller rays extending nearly to beak b. 



b. Small rays several 346. A. occidaneus. 



b. Small rays one in broad space 347. A. parvulus. 



II. Without rays, marked only by concentric lines 344. A. interlineaius. 



. Small, less than 1 inch in greatest diameter III. 



III. Radiating striae fine and numerous 4. 



4. Radii crossed by fine concentric lines, producing a cancellated appearance.. c. 



c. Length and height nearly equal (about \ inch) — 342. A. pellucidtis. 



c. Height slightly exceeding length (about f inch wide). 



338. A. cancellatus. 



4. Concentric striae absent or not prominent d. 



d. Length and height nearly equal 339. A. caroli. 



d. Height exceeding length 336. A. striatus. 



III. Radiating striae rib-like and alternating in length 5. 



5. Length and height about equal e. 



e. Posterior margin nearly straight 341. A. rectilaierarius. 



e. Posterior margin angular 340. A. coxanus. 



5. Height exceeding length 350. A. germanus. 



333. A. fasciculatus Hall. (Fig. 650, g.) Devonic. 



Axis of shell quite oblique to the hinge line. Surface including 



