512 TRANSACTIONS OF THE WAGNER FREE 



age remarkable and persistent specializations. Though most of its sub- 

 divisions have arrived at a notable degree of distinctiveness, intermediate forms 

 of ancient date connect them all, more or less effectively, with the parent 

 stem. 



Order ANOMALODESMACEA. 



Pelecypods having the mantle lobes more or less completely united, leav- 

 ing two siphonal, a pedal, and sometimes a fourth opening between them ; 

 siphons well developed, always at the posterior end of the body ; two sub- 

 equal adductor muscles ; the shell structure nacreous and celliilo-crystalline, 

 rarely with a prismatic layer; the area amphidetic or obscure, rarely distinctly 

 divided ; the ligament usually opisthodetic, generally associated with an in- 

 ternal resilium, chondrophores and lithodesma; valves generally unequal, the 

 dorsal margin without a distinct hinge-plate, the armature of the hinge feeble, 

 often obsolete or absent ; rarely with lateral lamina; or well-developed dental 

 processes ; the animals usually sedentary burrowers, hermaphrodite and marine. 



This group is intimately related to many Paleoconcha, except in the 

 presence of a pallial sinus. It retains many archaic characteristics, and in- 

 cludes several of the most peculiarly specialized modern forms. Through the 

 Anatinacea it approximates to the Myacean Teleodesmacea. It is peculiar in 

 the possession of a lithodesma, and in the structure of its gills and hinge. 

 The forms with a reticulate gill have it of a different type from the reticulate 

 gills of the other orders ; those which retain a modified foliobranch gill have 

 it different from the foliobranch gill of Prionodesmacean groups. There are 

 no forms with a filibranchiate gill, or with a typically fully-developed reticulate 

 gill. 



Order TELEODESMACEA. 



Pelecypods with reticulate gills, the ventricle of the heart embracing the 

 rectum ; having the lobes of the mantle generally more or less connected and 

 usually possessing developed siphons ; the adductors practically equal ; the 

 shell structure cellulo-crystalline (porcellanous) or obscurely prismatic, never 

 nacreous ; the dorsal area, if present, always prosodetic or divided into lunule 

 and escutcheon ; ligament opisthodetic, with or without separate resilium ; 

 without a lithodesma, rarely with external accessory shelly pieces; armature 

 of the hinge characterized by the separation of the hinge- teeth into distinct car- 

 dinals and laterals, the posterior laterals when present are behind the ligament ; 

 the animals active or nestling, sometimes sessile, but rarely sedentary burrow- 

 ers, rarely inequivalve, usually possessing a hinge-plate and a pallial sinus. 

 The sexes usually separate. This group is doubtfully represented in the Paleo- 

 zoic rocks and especially below the Carboniferous, though foreshadowed by 

 some of the Paleoconcha. Though many of them live imbedded in the surface 

 of the sea-bottom, they are more or less migratory and only a few extremely 



