560 TRANSACTIONS OF THE WAGNER FREE 



palpi normal, not united behind ; foot rudimentary, not grooved or byssiferous ; 

 siphons greatly prolonged, united nearly to their ends, naked, bearing distally 

 a pair of calcareous shields or " pallets," moved by a special set of muscles ; 

 mantle edges smooth, united, except at the pedal foramen ; anterior adductor 

 degenerate, attached on the anterior edges of the valves and covered only by 

 the mantle ; dioecious, the larval young retained for a time within the branchial 

 cavity of the parent ; boring, chiefly in wood ; usually marine, rarely fluviatile. 



Shell much reduced, equivalve, auriculate, widely gaping, the valves ap- 

 posited ventrally only on the surface of a parietal tubercle ; adductor scars un- 

 equal, the anterior marginal, very small ; pallial line coincident with the valve 

 margins ; a styloid myophore projecting from the cavity of the beaks ; mantle 

 secreting a calcareous lining to the burrow; pallets variable in form, the valves 

 without attached accessory shelly plates; area none, hinge-margin reflected, 

 edentulous, the ligament absent or obsolete. 



Carboniferous? Jurassic to recent fauna. 

 Ex. Teredo, Calobates, Seplaria, Xylotrya, Nausitoria. 



A species of this group has been found in the Ganges, and another in the 

 Zambesi, in perfectly fresh water, hundreds of miles from the sea. The paleo- 

 zoic species are known only by burrows, which are of somewhat doubtful 

 origin. 



In conclusion the reader is reminded that this summary represents not 

 the entire truth in regard to the groups characterized, but only an approxima- 

 tion to our present knowledge of them. The marshalling of the characters 

 here given will doubtless do much to call attention to discrepancies and errors 

 hitherto unchallenged, and, by its very defects, lead to an amelioration of the 

 system. 



