JUKES-BKOWNE : CEETACEOtTS AND EOCENE VENEEID^. 169 



differs from dementia in the absence of any pallial sinus, and if we 

 bear in mind the great variability of the sinus in the Cyprimeria- 

 Cyclorisma group (see p. 166), the absence of a sinus will not prevent 

 our regarding Psathtira as a subgenus of dementia. If this broader 

 view of the genus dementia be accepted, its definition must be 

 modified so as to include Flaventia and Psathiira as subgenera. This 

 can be done as follows : — 



Shell transversely oval, inequilateral, convex, concentrically striated 

 or undulated, without defined escutcheon and without lunule (except 

 in Flaventia) ; pallial line deeply sinuated (except in Psathura) ; 

 hinge bearing three cardinal teeth in each valve ; in the right the 

 anterior and median are nearly vertical, the posterior oblique and 

 bifid ; in the left there are three diverging teeth. 



dementia (sensu stricto). — Shell thin, fragile, white. Pallial sinus 

 deep, ascending, rounded. Right posterior tooth short, straight, and 

 narrow, consisting of two parallel laminae ; the left median thicker 

 than the others. 



Flaventia. — Shell fairly strong, with a defined lunule. Right 

 posterior tooth long and deeply bifid, the two laminae being of unequal 

 length. 



Psathura. — Shell thin and fragile. Hinge like dementia, the right 

 posterior very short, left valve with three equal and entire teeth. 

 Pallial line entire. 



15. Meecimonia, Ball. PI. VI, Fig. 11. 



This is a group of small shells which have hitherto only been found 

 in the Eocene of the Paris Basin. They were included in the ' Vetms ' 

 of Deshayes, and were referred to Mercenariahj'K. Cossmann in 1886, 

 but were distinguished by Dr. Dall in 1902 under the name of 

 Mercimonia (op. cit., p. 361), Venxf,s Bernayi, Cossm., being taken as 

 the type. 



Mercimonia differs from Mercenaria and diione in several respects, 

 and notably in having smooth inner margins. Dr. Dall regards it as 

 a section of Katelysia, which he located as a subgenus of his Marcia 

 group. There is certainly much resemblance between Marcimotiia 

 and Marcia, but I can see little between the former and Katelysia, 

 which I should not place in the same genus, because it has two 

 grooved or bifid teeth in each valve, and is more nearly akin to Tapes 

 and Hemitapes. 



It is a question whether Venus exalhida, Dillw., is the rightful type 

 of Marcia^ but this is the shell with which Mercimonia may be best 

 compared, and it is briefly desciibed by Dall as a Venus (i.e. Mercenaria) 

 " without hinge rugosities, radial sculpture, or marginal crenation." 

 Mercimofiia, however, possesses some special characters of its own, and 

 for the present I prefer to let it stand by itself as a separate genus, 

 until its afiinities are better understood and the ' Marcia ' question has 

 been satisfactorily settled. 



M. Cossmann gave no general description of Slercimonia, though 

 he described M. Bernayi in detail, and Dr. Dall's definition was only 



VOL. VIII. — OCTOliEK, 1908. 14 



