532 TRANSACTIONS OF THE WAGNER FREE 



Shell very inequivalve, free or sessile, solid, siibnacreous, edentulous, the 

 dorsal margins of one valve overlapping those of the other, which fit into 

 corresponding depressions in the shell wall ; ligament amphidetic, external or 

 absent; resilium internal, alivincular; area amphidetic or obsolete, a false area 

 formed on each side of the beaks by the flattened cardinal margin of the 

 valves ; shell closed ; pallial sinus small. 



Tertiary and recent faunas. 



Ex. Myochania, Myodora. 



The gills and siphons o{ Myochmna are more like those of Thracia than 

 of the PandoridcE, with which it has usually been associated. The anatomy of 

 Myodora is unknown. Its minute area curiously recalls that of Spondyhis. 



B. Adelosiphonia. 

 Siphons short, the lithodesma dividing the mass of the cartilage mesially. 



Family PANDORID^. 



Gills with the external direct lamina small or obsolete, free ; gills united be- 

 hind to form a septum ; foot small, compressed, grooved, not byssiferous ; opis- 

 thopodium and opisthopodial orifice absent; pedal foramen anterior, large; 

 siphons short, separate, papillose ; branchial orifice without a conspicuous cur- 

 tain valve, the tubes contractile only; monoecious, marine. 



Shell compressed, inequivalve, free, solid, with nacreous and prismatic 

 layers; the dorsal edges of the valves overlapping but not socketed, with denti- 

 form crural ridges on either side of the resilium, but no true teeth ; ligament 

 amphidetic, external, obsolete ; resilium internal, opisthodetic, usually reinforced 

 on its anterior surface by a mesial elongate lithodesma ; area none ; valves 

 closed, beaks entire, pallial line simple. 



Cretaceous to recent fauna. 



Ex. Pandora, Ctidiophora, Cce/odon. 



Pandora Wardiana A. Adams has no trace of an external direct gill 

 lamina ; in P. inequivalvis L. a narrow strip is present. 



Family LYONSIID^. 



Gills folded, as usual, united and septary behind, attached to the dome of 

 the mantle and to the siphonal septum ; foot compressed, grooved, byssiferous, 

 without an opisthopodium, but with an opisthopodial orifice ; pedal orifice 

 small ; siphons short, separate, papillose, branchial with the valve inconspicu- 

 ous ; moncecious, marine. 



Shell inequivalve, thin, subnacreous, edentulous; the ligament obsolete, the 

 resilium internal, uniting the edges of a long, mesial lithodesma to a narrow 



