INSTITUTE OF SCIENCE, PHILADELPHIA. 521 



valves usually closefitting ; gills reticulate with direct and reflected laminae, at- 

 tached to the mantle and to each other ; mantle lobes free, without siphons ; 

 ventricle generally ventral to the rectum with a single (anterior) aorta ; otocyst 

 absent (? ); rectum sessile at the anal end. 



FAMILY OSTREID^E. 



Shell distorted by early adherence to other objects ; monomyarian, the 

 anterior adductor absent ; edentulous, or with obscure schizodont dentition ; 

 dimyarian when young ; the foot obsolete or absent in the adult. 



Carboniferous to recent fauna. 



Ex. Oslrea, Gryphcea, Exogyra, Chalmasia. 



As in all cases where the adult is sessile, there is a development of adap- 

 tive characters as well as degeneration. The species are monoecious or dice- 

 cious and the position of the ventricle varies. Only the fossil forms exhibit 

 any trace of teeth, or foot. Jackson and others have shown that Ostrea is an 

 offshoot from the Pteriid/z rather than the Pectinida, which is further confirmed 

 by Menegaux's investigations. Many of its peculiarities are due to the lateral 

 position fixed by the sessile habit and these are common to most other sessile 

 forms. 



FAMILY ELIGMID^E. 



Shell thick, subequivalve, free when adult, resembling Chalmasia\n form, 

 anteriorly with an irregular pedal gape ; edentulous, monomyarian, with the 

 adductor seated on the free extremity of a myophore projecting from the um- 

 bonal cavity ; otherwise like Ostreidce. 



Oolitic. 



Ex. Eligmus (Nayadinaf). 



If the characters of Eligmus have been correctly interpreted, it can hardly 

 be retained in the Ostreidce. Perhaps further investigation of the genus would 

 be profitable. 



Superfamily NAIADACEA. 



Shell of varied form, normally equivalve, inequilateral and dimyarian ; 

 rarely alate ; shell substance nacreous and prismatic, with a conspicuous epi- 

 dermis ; area obscure or amphidetic ; ligament parivincular, usually opisthodetic 

 and external ; ventricle embracing the rectum, with anterior and posterior aor- 

 tas ; gills reticulate, with direct and reflected lamina.-, one or both pairs fre- 

 quently utili/.ed as a marsupium ; pleural ganglia fused with the cerebral ; oto- 

 cyst impervious ; pallial lobes usually free, except for an anal siphon, the pallial 

 line simple ; foot normally long, compressed, keeled, the byss us obsolete ; anal 

 end of rectum adherent ; young usually with a distinct nepionic stage ; station 

 usually fluviatile. 



