MOLLUSCA—PELECYPODA. 369 



are mostly similar and form a more or less continuous series g. 



g. Shell nuculoid, i. <?., hinge line rather sharply arched ; the small shell con- 

 centrically striate or smooth ; teeth numerous, similar and transverse.. 1 1 1 . 



in. Pallial sinus absent ; shell usually subcircular oo 



00. Teeth interrupted beneath the beaks by a resiliifer. 



XXVI. Nucula. 



00. No resiliifer present I2f. 



I2f. Shell transversely elongate XXVIII. Paleoneilo. 



I2f. Shell subcircular or vertically elongate... XXV. Ctenodonta . 



ill. Pallial sinus present ; shell transversely elongate .pp. 



pp. An internal vertical partition beneath the beaks. 



XXVII. Nuculites. 



pp. No internal partition 13-j-. 



I3f. Pallial sinus shallow XXIX. Leda. 



I3f. Pallial sinus deep XXX. Yoldia. 



g. Shell arcoid, i. e., hinge line straight or slightly arched ; teeth more or 



less vertical or with laterals subparallel to the hinge margin 222. 



222. All teeth vertical XXIV. Buchiola. 



222. All teeth vertical to hinge line except the end ones which are more 



or less oblique qq. 



qq. Hinge area with ligament grooves converging from the hinge 



margin to the beak 14-f. 



14-j-. Shell transversely elongate. 



XLIII. Area and XLIIIA. Barbatia. 



I4f. Shell subtrigonal XXXIV. Trigonarca. 



14-f. Shell subcircular XLIV. Glycimeris. 



qq. Hinge area with ligament grooves vertical... XXXV. Breviarca. 



222. Lateral teeth subparallel to hinge line rr. 



rr. Only posterior laterals present XXXI. Parallelodon. 



rr. Both anterior and posterior laterals present 15-f. 



15-f. Shell transversely elongate XXXII. Nemodon. 



15-j-. Shell subquadrate or subcircular XXXIII. Cucullcea. 



g. Shell cyrtodontoid, i. e.> hinge line straight or only slightly arched ; teeth 

 not forming a continuous series but the posterior laterals widely sepa- 

 rated from the cardinals ; no anterior lateral present. All teeth more 

 or less parallel to hinge margin. Strongly convex, subcircular shells 



with prominent umbos 333. 



333. Cardinal teeth two or more ss. 



ss. Cardinals behind the beaks XXXVIII. Ortonella. 



ss. Cardinals in front of the beaks i6f. 



l6f. Anterior adductor scar deeply prominent 13*. 



13*. Anterior adductor scar excavated out of hinge-plate. 



XXXIX. Vanuxemia. 

 13*. Anterior adductor scar free from hinge-plate. 



XXXVII. Megambonia. 



i6f. Adductor scars subequal 14*. 



14*. Area posterior to beaks (escutcheon) striated. 



XL. Whitella. 



14*. Area posterior to beaks not[striated.. XXX VI. Cyrtodonta. 



333. Cardinal tooth one (a strong cardinal process). .XLI. Plethocardia. 



Hinge not taxodont //. 



