XYLOPHAGA. 91 



nearly vertical, slightlj-diverging, approximate costellee, 

 which radiate from the anterior side of the umbones to the 

 greatly arcuated ventral margin, and vary greatly in size 

 and elevation in different examples. The hinder is, how- 

 ever, invariably the most distinct, the anterior one being, in 

 some specimens, almost entirely obsolete. The narrow space 

 between them is usually depressed ; which is also not un- 

 frequently the case with that portion of the shell which 

 immediately succeeds the posterior rib. The perfect smooth- 

 ness of the general surface is relieved by the fine and 

 crowded elevated lines which striate the front triangular 

 area and- the strip which lies between that and the anterior 

 costella ; those of the former (whose base is scarcely con- 

 vex, and but little oblique) are concentric, and closely set ; 

 those of the latter, which are delicate, crowded, and not at 

 all crenulated (as in the Teredines)., run obliquely down- 

 wards, diverging slightly as they advance. The umbones 

 are broad, and very prominent. The dorsal edge, which is 

 scarcely at all sloping, is very short, and reflected in front, 

 and nearly straight (but, in the young, tumid) behind. The 

 posterior side is simply and exactly rounded in the young ; 

 but in fine specimens is slightly subangulated at the upper 

 extremity. The rectilinear lower margin of the anterior 

 side, (which, as in Teredo, is furnished with a subrect- 

 angular incision, leaving a triangular area near the beaks,) 

 contrary to its direction in that genus, is nearly vertical, 

 almost forming a rectangle with the ascending ventral, 

 which latter being more tumid posteriorly, renders the con- 

 tour of the valves a little oblique. The hinge margin pre- 

 sents a single, distinct, and curved, tusklike denticle, just 

 beneath the beaks, which, springing from beneath the 

 margin in one valve, leans over to the opposite one. 



The accessory plates, which, however, are rarely present 



