XYLOPHAGA. 119 



crested ridges, which are pectinated at the edges, and separate 

 at the extremities : foot large, pillar- shaped, capable of being 

 protruded to some length. 



SHELL globular, semitransparent, and somewhat glossy, 

 divided lengthwise by a double ridge and furrow, which latter 

 is terminated inside by a small knob or tubercle in the middle 

 of the front edge ; anterior part triangular and sculptured by 

 numerous fine transverse striae ; middle area or strip narrow 

 and covered with oblique, finer and more crowded striae ; 

 posterior part on the other side of the ridge nearly smooth, 

 and having the end closed : leaks as in the last genus : apo- 

 physes short and prominent : dorsal shields two, similar to 

 those in Pholadidea, but proportionally much larger and more 

 conspicuous as well as more complicated in structure. 



Although Xylophaga resembles Teredo in the shape 

 and sculpture of its valves, and forms a connecting link 

 between the Pholadida and Teredinidte, it is more 

 nearly related to the former than to the latter family. 

 Its habits are those of Pholas, in never perforating wood 

 or vegetable matter (its only habitat) to a much greater 

 depth than is necessary for the reception of its shell. 

 It has no testaceous sheath or pallets like Teredo ; but, 

 instead of these processes, its shell is provided with 

 dorsal shields or plates, similar to those possessed by 

 other members of its own family. In fact it is a short 

 Pholas, and not a long Teredo. More information as 

 to the animal is desirable : I believe it can be entirely 

 contained in the shell. The epidermis is conspicuous, 

 and closely invests the anterior side of the shell ; this 

 affords an additional proof that the valves in the present 

 case cannot be the instrument of excavation, otherwise 

 the epidermis would be the first thing to be removed, 

 from the continual friction to which that part must be 

 subjected. Only two species have been described, one 

 inhabiting the North Atlantic, and the other South 

 America ; both are recent. 



