PANAMIC-PACIFIC PELECYPODA 133 



Genus LITHOPHAGA Roeding, 1798 



Type species by monotypy, L. mytuloides Roeding (^Mytilus litho- 

 phagus Gmelin). 



Borers into rock or thick-walled mollusks, corals, and the like, the 

 shell elongate, subcylindrical, rounded in front, the posterior end often 

 tapered, the beaks strongly anterior. Surface smooth except for lines of 

 growth and sometimes vertical or transverse striations. Hinge narrow, 

 edentulous, the ligament marginal, internal. Inner layer of shell nacreous. 

 Periostracum light brown or chestnut-colored, naked or covered with a 

 calcareous deposit, often becoming much thickened and produced beyond 

 the posterior end of the valves as spurs or blades. Margin of valves closed 

 in the adult. 



Like other members of the family, the young Lithophaga is fixed by its 

 byssus but it soon begins to bore into the substratum and developing 

 within the rock or a thick-walled shell, a large, often spacious, bullet-shaped 

 chamber, small at the top, the exit to the exterior forming a rounded or 

 key-hole shaped opening. In Leiosolenus, there is also an accessory, flattened, 

 calcareous tube which rises 2 to 5 mm., above the surface, its general ap- 

 pearance that of the elevated end of a Vermetus or worm-tube. Except in 

 Lithophaga, s.s., the surface of the valves is covered by a calcareous en- 

 crustation which often becomes much thickened along the posterior slope, 

 sometimes developing a plumose pattern there, and which may be pro- 

 longed beyond the ends of the valves in the form of thickened wedge-shaped 

 blades or solid, rounded spurs. These calcareous extensions serve to close 

 off the entrance to the burrow when the shell is pulled back. 

 In Rupiphaga, the upper end of the rock chamber is lined with a secondary, 

 thick-walled, cemented tube as do some of the rock-boring Pholades such 

 as Hastasia. Excavation of the lithophagoid burrow is accomplished ap- 

 parently by juices secreted by the animal, the softened debris removed by 

 water currents. The surface of the valves shows generally no abrasive wear 

 as do so many other rock-boring clams. 



Key to subgenera of Lithophaga 



Surface of the valves, smooth, clean and polished, without a coating of 

 lime, the periostracum visible throughout. 



Subgenus Lithophaga, s.s. 



Surface of valves covered by a secondary deposit of lime. 



I. Calcareous encrustation on the surface of the valves only, not pro- 

 longed beyond the posterior ends. Orifice of the burrow continued 

 above the surface as an elevated, accessory, calcareous tube, its open- 

 ing bilobate. 



Subgenus Leiosolenus 



II. Calcareous coating prolonged beyond the posterior end of the valves 

 as thickened blades or spurs. Orifice of burrow without an accessory 

 tube. 



