546 TRANSACTIONS OF THE WAGNER FREE 



re.silium and the cardinal formula |^—J-^-j, with the larger cardinal bifid, and 

 the teeth not soldered above. These can take the name Joannisia. The type 

 is J. oblonga Sowerby (sp.). The anatomy o'i Joannisia is still unknown, 



Family CRYPTODONTID^. 



Gills smooth, with internal and external direct and reflected laminae, the 

 external limb dorsally directed ; gills free behind the foot ; palpi obsolete ; 

 hepatic and generative glands external to the visceral mass, pedunculated, ar- 

 borescent ; foot slender, much elongated, as in Diplodonta ; mantle edges thick- 

 ened, smooth, ventrally free ; branchial foramen incomplete, anal not produced 

 into a tube ; dioecious, marine. 



Shell substance earthy, with inconspicuous epidermis and prosocoelous 

 beaks ; valves equal, free, closed, with plain margins, feebly concentrically striate 

 or smooth, usually with a radial posterior flexure ;■ adductors Liici7toid,^3.\- 

 lial area often punctate; ligament and resilium parivincular, opisthodetic.subex- 

 ternal, seated in a groove ; area impressed ; hinge feeble without lateral laminae, 

 edentulous or with an obsolete cardinal tooth in the right valve. 



Cretaceous to recent fauna. 

 Ex. Cryptodon, Axinus, Conchocele, Philis. 



The genus Montacjcta, with Lucinoid gills, has hepatic digitations like 

 those of Cryptodon, but in some other respects seems more like some of the 

 Leptonacea. A better knowledge of the latter is needed before the various 

 genera can be assigned a permanent place. 



Superfamily LEPTONACEA. 



The incurrent and excurrent openings between the mantle lobes at oppo- 

 site ends of the body, the former anterior. 



This group contains a great many commensal, nestling, or parasitic forms, 

 if independent usually very active, crawling like Gastropods on a subreptary 

 foot, and with the mantle edges more or less reflected over the valves. 



Family LEPTONIDtE. 



Gills normal, the outer limb smaller, sometimes absent; palpi moderate; 

 foot long, usually subreptary, grooved and byssiferous; anterior incurrent 

 siphon sometimes incomplete, posterior anal or excurrent siphon developed ; 

 mantle margins extended, often with tentacular processes and with a single 

 anterior sensory tentacle prominently developed, the edges of the mantle not 

 permanently reflected over the valves ; the nepionic young usually protected 

 within the maternal shell ; monoecious, marine, frequently commensal. 



Shell cellulo-crystalline with a periostracum ; valves equal, free, smooth- 

 edged, often gaping, variably sculptured ; adductor scars peripheral, subequal ; 

 pallial line simple ; area obscure or none ; ligament parivincular, opisthodetic. 



