INSTITUTE OF SCIENCE, PHILADELPHIA. 549 



serve as a fulcrum behind; mantle edges papillose; pallial line simple; anal 

 for .unen with a well-developed valve, but no siphon ; branchial large, usually 

 complete ; perisiphonal area profusely papillose ; sexes usually separate ; 

 marine. 



Shell substance cellulo-crystalline, with the external layer more or less 

 tubular ; with a variable epidermis ; valves equal, free, gaping slightly behind, 

 the beaks prosocculous, the margins usually serrate or radially striated ; adduc- 

 tor scars subequal, the pedal distinct and usually distant ; ligament and resil- 

 i u m parivincular, external, set in a groove, short; area obscure; the complete 

 armature of the hinge includes an anterior and posterior lateral in the left, and 

 tuo anterior and one posterior lateral in the right valve (any or all of which 

 may be absent), the cardinal formula is jf^TTl' tne teeth simple, smooth, never 

 bifid, one cardinal in each valve usually persistent, the others inconstant. 



Triassic to recent fauna. 



K.\. Pro/ocardia, Leptocardia, Cardium, Fragum, Cardissa, Lophocardium, Hemicardiutn, 

 Papyridea, Liocardium, Serripes, fDidacna. 



In Hcinicardiiiin biangnlatum the branchial siphon is incomplete below. 

 In the details of the peripheral anatomy this group is very variable. One of 

 the fossil forms is described as having the ribs of the shell perforated with rows 

 of small holes leading to the interior of tubular spines (Etkmoeardium), but it 

 is probable these did not communicate with the exterior. 



FAMILY ADACNID^E. 



Like Cardiidtc, but with long united siphons, a short compressed foot, the 

 shell thin, with a pallial sinus and obsolete hinge armature; living in brackish 

 or fresh water. 



Tertiary and recent faunas. 

 Kx. shiacna, Lhnnocardium,Arcicardium. 



This group has only recently diverged from the preceding family, with 

 which the fossils unite it by gradations. It is interesting to note how fre- 

 quently, in the descendant of short-siphoned marine forms, the modifications 

 fitting it for life in fresh or brackish water include long and more or less 

 united siphons; as in the case of Dreissensia, Cyrenella and Adacna. 



Superfamily TRIDACNACEA. 



Soft parts rotated forward nearly 180 with relation to the valves as com- 

 pared with normal Pelecypods, the anterior adductor wanting, and the poste- 

 rior nearly central in the shell ; cardinal teeth lamellar, oblique. 



FAMILY TRIDACNID^E. 



Gills deeply plicate, the exterior limb smaller, not dorsally directed nor 

 appendiculate, united behind; palpi as in Cardiidce ; foot small, short, byssifcr- 



