134 AXEL A. OLSSON 



A. Calcareous extension at the posterior end of one valve only. Rock 

 burrow lined by an inner, thickened, cemented tube. 



Subgenus Rupiphaga 



B. Calcareous extensions at the posterior end of each valve. 



1. Posterior calcareous extension in the shape of alternate, overlapping 

 or twisted blades. Orifice bilobate. 



Subgenus Myojorceps 



2. Calcareous extensions, straight, equal, not twisted or alternate. 



a. Calcareous extensions rectangular or wedge-shaped. Encrustation along 

 the posterior slope, smoothish or merely granulose. 



Subgenus Labis 



b. Calcareous encrustation along the posterior slope thick and heavy 

 and with a plumose surface pattern. 



Subgenus Diberus 



Subgenus LITHOPHAGA, s.s. 



Surface of valves without a calcareous encrustation. 

 No known species in the Panamic-Pacific faunal area. 



Subgenus LABIS Dall. 1916 



Type species by monotypy, Lithophaga attenuata Deshayes. 



Surface covered by a thin, calcareous incrustation, thickened at the 

 posterior end and prolonged beyond the valve margin into a short, rec- 

 tangular wedge or blade. 



Lithophagra (Labis) atti*nuata (Deshayes) Plate 15, figures 3-3c, 4 



Modiola attenuata Deshayes, 1S36, in Lamarck's Anim. s. Vert., vol. 7, p. 28. "Habite 



au Perou, au Chile, dans les pierres." — Sovverby, 1824, Gen. Shells, Lithodomus, 



fig. 3. 

 Lithodomus attenuatus (Deshayes), Chenu, 1862, Manuel de Conchyliologie, vol. 2, 



bivalves, p. 156, fig. 767. 

 Lithophaga (Labis) attenuata (Deshayes), Hertlein and Strong, 1946, Zoologica, vol. 



31, pt. 2, No. 5, pp. 74, 75. — Soot-Ryen, 1955, Allan Hancock Pacific Expeditions, 



vol. 20, No. 1, pp. 99, 100, pi. 10, fig. 57; text-fig. 77. 



Shell narrowly elongate, subcylindrical, evenly convex over most of 

 its length. Dorsal and ventral margins straight for the most part and 

 parallel except for the posterior half where the dorsal margin descends to 

 form a long-drawn out posterior end tipped by a thick, calcareous wedge 

 or flattened blade. The posterior-umbonal slope is slightly flattened, 

 bordered on the ventral slope and sometimes above by an impressed line, 

 the lines of growth across it showing in a rectangular pattern. Perio- 

 stracum thin, brownish colored and covered by a calcareous encrustation 

 which is much heavier on the posterior slope, prolonged beyond it, as a 

 heavy flattened blade. 



Length 95 mm.; height 19.2 mm.; diameter 18 mm. Manta, Ecuador. 



This is a common rock borer at many places and at Manta, Ecuador, 

 it is gathered as a sea-food but is said to have a strong, peppery flavor. 

 The calcareous blades at the posterior end of the valves are coarse and 

 thick, and these protuding slightly at the orifice of the burrow indicate the 

 presence of the clam within; these blades on being touched lightly induce 

 the clam to pull in at once. 



