INSTITUTE OK SCIENCR, PHILADELPHIA. 557 



Shell solid and heavy, usually donaciform, with erect or opisthogyrate 

 beaks, othrrwi.se as in Mactrida ; dicecious, marine or estuarine. 

 Tertiary to recent fauna. 

 The group may be divided into three subfamilies : 



ir ; Ex. l\f.ictrof>sis, Atactodea.Mesodesma, Donacllla, Taria, Paphies. 

 Da-ci lines ; Ex. Dai'ila, Anapella. 

 Erviliiiiir ; Ex l''.>vilia, Cfcce/la. 



E. Asthenodonta. 



Hinge often essentially Mactroid, but usually degenerate or obsolete, owing 

 to modifications due to the burrowing habit. These secondary adaptive modi- 

 fications have led to the association of the following types with the genetically 

 distinct Anomalodesmacea, which also exhibit them and for the same reason. 



Superfamily MYACEA. 



Burrowing, long siphoned, frequently inequivalve Pelecypods, usually with 

 the mantle lobes largely united below, more or less united siphons and degen- 

 erate hinge apparatus. 



FAMILY MYACID/E. 



Gills normal, plicate, usually not extended into the cavity of the branchial 

 siphon, united behind and forming an anal chamber ; palpi large, more or less 

 united behind ; foot small, grooved, byssiferous when young, but not in the 

 adult; mantle edges smooth, thick, duplex, largely united; siphons united, 

 with a horny tunic, not wholly retractile; no sensory lamina: behind the 

 branchial foramen ; anal end of the rectum free ; dkecious, marine. 



Shell substance cellulo-crystalline, earthy, with a conspicuous epidermis; 

 valves unequal, more or less elongate, rounded in front and gaping behind ; 

 adductor scars subequal ; pallial line sinuated ; shell margins plain ; area obso- 

 lete or none ; ligament and resilium internal, opisthodetic, attached in the left 

 valve to a projecting chondrophore, merging with the dorsal margin behind, and 

 in the right valve to an inconspicuous, usually subumbonal, chondrophore ; 

 hinge edentulous. 



Tertiary and recent fauna. 

 Ex. Mya, Cryptoinya, P/afyodon, Sphenia, Tugonia. 



Sphcnia retains the byssus when adult and is a nestler, the others are 

 burrowers ; in Tugonia, of which the anatomy is unknown, the chondrophore 

 of the right valve is well developed. 



FAMILY CORBULID^. 



Gills short, arranged as in Mya ; foot compressed, grooved, often byssifer- 

 ous ; mantle edge papilliferous, largely united ventrally ; siphons short, united, 

 with papilliferous tips, naked, wholly retractile; anal end of rectum sessile; 

 dioecious, marine or estuarine, rarely fluviatile. 



