INSTITUTE OF SCIENCE, PHILADELPHIA. 545 



into a siphon, but provided with a retractile valve ; shell differing from Lucini- 

 dce in being transversely oval, thick, with a heavy hinge-plate, and usually well- 

 developed laterals ; two or three strong cardinals in each valve; the margin of 

 the valves denticulate and the exterior strongly sculptured ; the adductor scars 

 are oval and do not project ventrally into the pallial area. 

 Triassic to recent fauna. 



Ex. Sphezriola, Sphtzra, Firnbriella, Corbicella, Corbis. 



This group is a peculiar offshoot from the Lucinida, with which it is closely 

 related and by many authors united ; the earlier members have the hinge-teeth 

 more or less variable or deficient. 



FAMILY DIPLODONTID^. 



Soft parts like Lucinidce, but with the external limb of the gills developed, 

 reflected and sometimes appendiculate ; the anal foramen not tubular, the ad- 

 ductor scars continuous peripherally with the pallial line; hinge with the lat- 

 erals obscure or absent, and the valve margins plain ; the shell subcircular in 

 outline, rarely nestling and irregular. 



Jurassic ? Cretaceous to recent fauna. 



Ex Ungulina, Diplodonta, Axinopsis, Felania, ? Sphcerella, fTenea. 



Woodward represents the branchial foramen of Diplodonta as complete, 

 but Fischer places it with those Lucinacea, which have it incomplete. The 

 ligament in some forms, owing to the depth of the groove in which it is set, 

 becomes partially internal. The name Ungitlinidce has been used for very 

 different assemblages of genera, and I prefer to use a name for the family about 

 which there can be no uncertainty. 



FAMILY CYRENELLID^E. 



Gills with both inner and outer direct and reflected laminae, the outer 

 limb smaller, united behind as in Ludna ; palpi elongated ; foot elongate cla- 

 vate; mantle edge plain ; both siphons developed, elongate and united to their 

 tips, contractile ; dioecious, inhabiting fresh or brackish water. 



Shell as in Diplodonta, but with a conspicuous epidermis ; pallial area 

 smooth; pallial line not sinuate; hinge without lateral laminae; the cardinals 

 like Diplodonta or with two cardinals in each valve soldered to each other dor- 

 sally ; cardinal formula i \ \ \ \ J. the anterior left cardinal usually obsolete. 



Pliocene to recent fauna. 



Ex. Cyrenoida, Joannisia. 



These are estuarine Lucinacea. The species of Atlantic shores form the 

 typical and a homogeneous group. Those described from the Philippines by 

 Sowerby are different, have the hinge of Diplodonta with a deeply-immersed 



