INSTITUTE OF SCIENCE, PHILADELPHIA. 535 



gills, and to the substitution of a fleshy or muscular partition in the place of 

 the ordinary branchial septum. 



Family EUCIROID^. 



Ventricle dorsal to the rectum with a single anterior aorta ; foliobranchiate 

 with the laminae extended laterally, each with a lateral equivalent of a re- 

 flected lamina attached to the mantle and to the siphonal septum ; labial 

 palps developed ; foot digitiform, grooved, not byssiferous, without an opistho- 

 podium; the pedal foramen large ; siphons short, separate, the branchial large 

 without a conspicuous curtain valve ; monoecious, marine. 



Shell subequivalve, nacreous and cellulo-crystalline, externally granulose ; 

 hinge with a strong tubercle in the right valve before the resilium and the dor- 

 sal margins modified to over and underlie each other ; ligament obsolete; resil- 

 ium opisthodetic, internal, with a strong lithodesma ventrally ; area obscure or 

 absent; a depressed false lunule before the beaks; valves closed, pallial sinus 

 shallow, obscure. 



Tertiary to recent fauna. 

 Ex. Pecchiolia, Euciroa. 



Family VERTICORDIID^. 



Ventricle embracing the rectum, probably with two aortas ; gills folio- 

 branchiate, simple, without a lateral reflected extension, adnate on the an- 

 teriorly extended fleshy imperforate siphonal septum ; palps obsolete; foot 

 digitiform, not byssiferous; without an opisthopodium ; pedal foramen small; 

 siphons short, separate, papillose ; branchial without a conspicuous valve ; 

 monoecious ? marine. 



Shell as in Enciroidce, variable. 



Tertiary to recent fauna. 

 Ex. Verticordia, Trig07mlina, Haliris. 



Family POROMYIDvE. 



Ventricle embracing the rectum, with two aortas ; gills degenerately folio- 

 branchiate, lamella; planted in patches symmetrically on a muscular septum, 

 separating the anal and branchial chambers, each patch consisting of a few 

 close set short lamellae arranged in a single row with orifices between them lead- 

 ing into the anal chamber, or reduced to simple protuberant lips surround- 

 ing single perforations of the septum ; palps present, large ; foot small, grooved, 

 pointed, not byssiferous, without an opisthopodium ; siphons short, separate, 

 the branchial with large papilljE and a prominent curtain valve ; monoecious, 

 marine. 



Shell subequivalve, rounded, nacreous and cellulo-crystalline, granular or 

 smooth externally ; hinge with obscure tubercles in front of the resilium and 

 the dorsal edges of the valves as in VerticordiidcB. Ligament external, opistho- 



