PELECYPODA—PRIONODESMACEA. 467 



LXXX. Gryph^ea Lamarck. 



Sessile when young, free when adult. Left valve strongly- 

 arched, with beak incurved usually nearly in plane of median line 

 and usually not much deflected to right or left. Right valve flat 

 and opercular ; its beak inconspicuous and not deflected or coiled. 

 Surface usually marked only with concentric growth lines. Muscle 

 impressions as in Ostrea. Less irregular than Ostrea, with beak 

 of lower valve more produced and recurved. It usually also 

 lacks the strong radial plications often seen in Ostrea and Exogyra. 

 Also differs from Exogyra in that the beak of the lower valve 

 curves upward instead of laterally. Young Grypkceas are often 

 much like Ostreas. Jurassic-Tertiary. 



A. Shell narrow, widening but little from beak I. 



I. Dorsal sinus present I. 



I. Dorsal sinus beginning near beak 290. G. mucronata. 



1. Dorsal sinus beginning some distance from beak 286. G. marcoui. 



I. Dorsal sinus absent 285. G. mexicana 



B. Shell broad, widening rapidlyfrom beak II. 



II. Dorsal keel present 288. G. navia. 



II. Dorsal keel absent 2. 



2. Lower valve winged a. 



a. Wings on both sides II. 



11. Small (less than 2 inches high) 289. G. washitaensis. 



11. Large (over 2 inches high); lower valve nearly circular, slightly 

 compressed into wings at cardinal extremities. 



292. G. mutabilis. 



a. Wing only posterior 22. 



22. Wing not separated from rest of shell by deep sulcus ; surface 



marked with obscure concentric lines 291. G. vesicu/aris. 



22. Wing separated from rest of shell by deep sulcus ; surface marked 



with rugose concentric lines 293. G. convexa. 



2. Lower valve not winged b. 



b. Beak usually somewhat twisted 287. G. corrugata and vars. 



b. Beak straight 294. G. newberryi. 



285. G. mexicana Felix. (Fig. 622.) Upper Jurassic. 

 Left valve with strongly curved beak which is flattened poster- 

 iorly. Growth lines usually thickened into wave-like elevations. 

 Right valve small, flat, and triangular. 



Malone of Texas ; also Mexico. 



286. G. marcoui Hill and Vaughan. (Fig. 623.) Comanchic. 

 Beak narrow and incurved. Dorsal sinus distinct. 

 Fredericksburg of Texas. 



