INSTITUTE OF SCIENCE, PHILADELPHIA. 559 



gle obsolete cardinal, or wholly edentulous ; animal frequently forming an ex- 

 ternal protective tube to supplement its burrow, but to which it is in no way 

 attached. 



? Permian, Triassic to recent fauna. 

 Ex. Gasirochizna, Rocellaria, Fistulana, Spengleria, Cucurbitula. 



This group stands between the Myacea and the Adesmacea, verging on 

 the latter. Many of its characters are adaptive and are repeated in the En- 

 siphonacea, but morphologically its relations to SaxicavidcE seem close. 



Superfamily ADESMACEA. 



Gills with direct and usually reflected laminae, long, united, extended into 

 the branchial siphon ; posterior adductor usually in front of the visceral 

 ganglion, anterior adductor external to the cavity of the valves, exerted in a 

 contrary sense to the posterior muscle ; hinge-margin reflected, edentulous ; 

 ligament obsolete; a myophoric process extending freely into the valve from 

 the subumbonal cavity. 



Family PHOLADID^. 



Gills not fused in the median line, extending well forward into the oral 

 region, palpi large, partly supported by the styloid myophore ; foot large, 

 modified in front as a triturating organ ; behind usually grooved but not bys- 

 siferous ; mantle lobes united except for the pedal orifice, smooth or papillose, 

 duplex at the margin, the inner fold conspicuous; anterior adductor covered 

 by the mantle and often by accessory shelly deposits ; siphons united to their 

 ends, naked or tunicate, without " pallets," largely retractile ; dioecious, marine, 

 boring in hard or solid substances, 



Shell cellulo-crystalline, with a thin epidermis ; valves more or less gap- 

 ing in front and behind, with inconspicuous beaks and reticulate, often spinose 

 sculpture, but not colored ; in the adult supplemented by accessory shelly 

 pieces, always attached to the valves, but not by an exterior shelly tube like 

 that of GastrochcznidcB ; the antero-dorsal margins more or less extensively 

 reflected, the postero-ventral approximated; pallial line sinuated ; area none; 

 ligament and resilium usually absent, an obsolete remnant of resilium and 

 chondrophore sometimes present in the left valve. 



Carboniferous to recent fauna. 



Ex. Turmis, Pholas, Zirphcsa, Talona, Pholadidea, Jouanneiia, Xylophaga, Mariesia, Par- 

 apholas, Teredina, etc. 



Family TEREDINID^. 



Pedal ganglia obsolete, the visceral behind the posterior adductor ; ven- 

 tricle of the heart ventral and posterior to the rectum, elongate, with a single 

 anterior aorta (equivalent to a fusion of the two aortas of ordinary forms) ; gills 

 fused in the median line, with direct but not reflected laminae, the external limb 

 reduced, situated behind the viscera and extended into the branchial siphon ; 



