BALL : MACTRIDiE AND MESODESMATID^. 209 



Subfamily LUTRARIIIS"^. 



Shell inequilateral, widely gaping ; hinge tending to be irregiilar, 

 concentrated ; the laterals partly reduced, or obsolete ; the cartilage- 

 pit free, in the plane of the hinge-plate. Siphons contractile, not 

 retractile within the shell, clothed with a horny epidermis to their 

 tips ; ventral opening of the mantle short, and the foot corresj)ondingly 

 reduced in size. 



Subfamily ZEKATIIN^. 



Shell inequilateral, compressed ; hinge concentrated, irregular, the 

 laterals tending to become obsolete, or absent ; chondrophore bent out 

 of the plane of the hinge-plate, and more or less adherent to the valve ; 

 siphons retractile ? Ventral opening of the mantle and foot much 

 reduced. 



? Subfamily ANATINELLIN^.i 



Shell inflated, gaping, radiately sculptured ; hinge with a prominent 

 narrow chondrophore, a short external ligament, a narrow cardinal 

 and an accessory lamella in each valve, without laterals ; pallial line 

 without a sinus ; soft parts unknown ; siphons probably short. 



Family MESODESMATID^. 



Retractile, naked, siphons more or less completely separated ; shells 

 cuneiform, more or less compressed, and disproportionately heavy. 



SubfamUy MESODESMATIN^. 



Shells with a pallial sinus, porcellanous texture, with a conspicuous 

 periostracum ; ligament inconspicuous, and, if wholly external, more 

 or less degenerate, otherwise as in Spisula ; resilium narrow, oblique ; 

 recent species with a single,' narrow, long, left cardinal tooth, with a 

 short posterior arm extended across the apex of the cartilage-pit ; 

 right cardinal lamella feeble, nearly obsolete in the recent species, 

 but normal in the Eocene forms. 



Subfamily DAVILIN^. 



Pallial line simple ; shells more rounded than in the preceding sub- 

 family, but the other characters similar ; siphons probably short. 



' I have not been able to examine the genus Cardilia. 



