370 NORTH AMERICAN INDEX FOSSILS. 



h. Shell distorted usually by adherence to foreign objects, heavy, irregular, 



usually strongly spiral 444. 



444. Teeth absent or very obscure //. 



//. Beaks small, not curved LXXIX. Ostrea. 



tt. Beaks prominent, strongly incurved 17-f-. 



I7f. Beaks curved in the median plane LXXX. Gryphcsa. 



17-f. Beaks curved to right or left of median plane 15*. 



15*. Valve (the larger) narrowing to beak with great sudden- 

 ness (looks like Capulus, gastropod )...L. Loxopteria. 

 15*. Valve (thelarger) narrowing slowly.XXXXI. Exogyra. 



444. Teeth very prominent, heavy ««. 



uu. Shell with prolonged and irregularly twisted umbo. 



CXLVII. Monopleura. 



uu. Shell with regularly spiral umbo in one or both valves l8f. 



i8f. Surface strongly lamellose CXLV. Chama. 



i8f. Surface not lamellose 16*. 



16*. Attached (left) valve with regularly spiral umbo (right 



valve operculum-like) CXLVI. Requienia. 



16*. Attached (right) valve coniform. 



CXLVII. Caprina, 

 CXLIX. Jchthyosarcolites, 

 CL. Coralliochama. 



uu. Shell (one or both valves) conical, elongate 19-f-. 



I9f. Attached valve conical, vertically ribbed ; free valve flat or 



conical CL. Radioliles. 



I9f. Attached valve coniform ; free valve spiral ; shell marked 

 with concentric growth lines, though longitudinally fibrous 



within CXLVIII. Caprina, 



CXLTX. Ichthyosarcolites, 

 CL. Coralliochama. 



h. Shell more or less distorted through boring habit ; a calcareous siphonal 



tube present 555. 



555. Tubes found in sand. No shell known. Tubes have transverse 



septa CLXXXIII. Polorthus. 



555. Tubes found in wood vv. 



vv. Valves with a prominent anterior notch CLXXXI. Turnus. 



vv. Valves trilobed CLXXXIL Teredo. 



h. Shell usually entirely free from distortion 666. 



666. Surface marked with radial ribs ww. 



ww. Shell elongate laterally, the greatest length parallel with the 



long, straight hinge line CXXXI. Pleurophorus. 



ww. Shell subtriangular « 20f\ 



2of. Shell very elongate-triangular LXII. Pinna. 



20f. Shell of medium length or short 17*. 



17*. Surface with rows of nodes XC. Trigonia. 



17*. Surface spinose 12". 



i2 n '. Byssal opening prominent LIII. Honeoyea. 



i2 f/ . Byssal opening absent CIII. Plicatula. 



17*. Surface free from spines and nodes 13" • 



ij // . Shell exceedingly thin ; lower valve flat and per- 

 forate near umbo for the byssus..CVII. Paranomia. 



