53 TRANSACTIONS OF THE WAGNER FREE 



become the Teleodesmacea, while another part, sheltered by peculiarities of sta- 

 tion and developing adaptive modifications, has lagged behind the others and 

 still indicates in some degree the ancient types from which it sprang. 



Superfamily ANATINACEA. 



Anomalodesmacea having reticulate V-shaped gills, of which the left 

 slope of the V is homologized with the inner direct lamina, the right slope 

 with the inner reflected lamina, the right cross-stroke with an inner appendicu- 

 lar extension, and the left cross-stroke with the outer direct lamina, of the 

 typical Teleodesmacean gill ;* not secreting a calcareous tube external to the 

 shell; having the rectum embraced by the ventricle of the heart, usually with 

 an anterior and posterior aorta. 



A. Eusiphonia. 



Siphons elongate, the lithodesma, when present, at the anterior end of the 

 internal resilium, and external to the mass of the resilium. 



FAMILY CEROMYID^. 



Shell slightly inequivalve, hinge with an obscure projection or edentulous, 

 the cardinal border of one valve covering that of the other valve, which is 

 supplemented by a sort of laminar nymph, the ligament subinternal between 

 them ; area inconstant or obscure ; pallial sinus present ; valves closed or slightly 

 gaping. 



Trias-Jura. 

 Ex. Ceromya, Gresslyia, Mactromya (pars). 



FAMILY PHOLADOMYID^E. 



Shell substance nacreous and cellulo-crystalline; gills completely united 

 behind, forming a septum below the anal chamber ; foot small, with an opistho- 

 podium ; siphons long, united to their tips, not wholly retractile, naked ; ven- 

 tral commissure of the mantle with a pedal and an opisthopodial foramen. 



Shell thin, equivalve, gaping, edentulous or with an obscure subumbonal 

 tubercle ; ligament and resilium external, opisthodetic, seated on nympha; ; 

 area obsolete or obscure, not amphidetic ; beaks entire ; pallial sinus well 

 marked ; marine. 



Trias to the recent fauna. 

 Ex. Pholadomya, Proeardia, Arcomya, Guniomya. 



The granulations of the surface, upon which the above have been divided 

 into two families, do not seem of serious importance, judging by the differ- 

 ences observable in such recent genera as Thracia and Poromya, some of 

 which are granular and others smooth. 



* These horologies are not perfectly satisfactory, but seem probably correct in the present state of our 

 knowledge. The description, given for the left, should be reversed for the right-hand gill. 



