544 TRANSACTIONS OF THE WAGNER FREE 



constant; cardinals one in tlie left and two in the right valve, or two in each 

 valve ; marine. 



Triassic and Cretaceous. 

 Ex. Tancredia, Heinidonax ,? Meekia. 



This group is unknown to me autoptically, but is considered of family 

 rank by Fischer, and regarded, with Isodonta, by Neumayr, as representing 

 an early group oi LitcuiidcB. 



Family UNICARDIID.E. 



Shell cordiform, equivalve, closed, concentrically striated; adductor scars 

 elliptical, the anterior longer ; pallial line simple ; margin of the valves smooth ; 

 ligament external, parivincular, seated in a groove ; with a developed grooved 

 hinge-plate bearing a single obsolete cardinal in each valve, or none ; marine. 



Carboniferous to Cretaceous. 

 Ex. Scaldia, Pseiidedmondia, Unicardium. 



Inserted here on the authority of Fischer and Neumayr, and with the 

 limits assigned by the former. 



Family LUCINID^. 



Gills smooth, with an internal direct and reflected lamina, without ap- 

 pendix, the external limb absent, united behind the foot, forming a complete 

 anal chamber; palpi very small ; hepatic glands contained within the general 

 visceral mass ; foot not grooved, with a terminal mucous gland ; mantle edges 

 usually plain, thick, free ventrally ; branchial foramen papillose, not tubular, 

 incomplete below ; anal complete, usually with a retractile tubular siphon ; di- 

 oecious, marine. 



Shell substance porcellanous or chalky, usually with inconspicuous or 

 dehiscent epidermis, rounded, variably sculptured ; valves equal, free, closed, 

 with low, prosocoelous beaks ; adductor and pedal scars adjacent or distinct, 

 the latter small ; anterior adductor elongated, largely within the pallial line, 

 which is not sinuate ; area within the pallial line often granular or punctate ; car- 

 dinal area small, often deeply impressed ; ligament and resilium subinternal, 

 set in a deep groove, but usually more or less visible externally ; hinge-plate 

 distinct ; lateral laminae distant from the cardinals, anterior and posterior in 

 the right, and corresponding sockets in the left valve ; cardinal teeth radial, 

 formula g-^TTI' '■^^ posterior tooth larger and often bifid, but any or all of the 

 teeth may be obsolete or absent. 



Devonian to recent fauna. 

 Ex. Paracyclas, Lucina, Myrtcsa, Codakia, Miltha, Divaricella, Gonodon. 



Family CORBID^. 



Soft parts as in Licciiiidis, but with doubly-fringed pallial margins, rounded 

 adductors peripherally in line with the pallial line, anal orifice not produced 



