458 NORTH AMERICAN INDEX FOSSILS. 



257. A. crassicollis Keyserling. (Fig. 607.) Comanchic. 

 Large, inflated. Concentric markings varying from irregular 



constrictions to strong plications. 



British Columbia, Alaska, eastern Greenland. Knoxville of 

 California and Oregon. 



258. A. piochii Gabb. (Fig. 608, a, b.) Comanchic. 

 Small, thin-shelled. Concentric undulations regular. 

 Knoxville : California, Oregon, Washington. 



258^. A. piochii var. ovata Stanton. (Fig. 608, c, d.) 



Comanchic. 



Larger than A. piochii, more broadly triangular, and more 

 convex. 



Knoxville : California, Oregon, British Columbia. 



LXXIX. Ostrea Linneus. 

 Shell distorted by adherence to other objects. Structure 

 lamellar. Inequivalve, fixed by the left or larger valve. Beaks 

 terminal. Left valve convex, often marked with radiating ribs. 

 Right valve flat or concave, often smooth. Sculpture usually dif- 

 ferent on the two valves. Teeth generally absent. Anterior 

 muscle impression absent; posterior nearly central. Ligament 

 cavity conspicuous, triangular or elongate. Pallial line indistinct. 

 Carbonic to Recent. 



A. Surface conspicuously plicate radially I. 



I. Outline subcircular I. 



I. Plictions over whole valve 267. O. lugubris.. 



1. Plications only marginal 265. O. panda. 



I. Outline sickle-shaped, i. e., curved laterally 2. 



2. Entire shell plicate a. 



a. Plications 4 262. O. quadriplicata. 



a. Plications more than 4 271. O. falcata. 



2. Median area not plicate b. 



b. Shell of medium size 273. O. nasuta. 



b. Shell small II. 



II. Shell auriculate 272. O. mesenterica. 



II. Shell not auriculate 279. O. pellucida. 



I. Outline triangular 260. O. crenulimargo. 



I. Outline oblong-ovate with height much exceeding transverse diameter. 



261. O. subovata. 



B. Surface not conspicuously plicate radially except in some forms when young II. 



II. Both valves deeply sinuous 281. O. sellceformis. 



II. Valves not sinuous 3- 



3. Lower valve very convex ; upper valve flat or only slightly convex c. 



