PANAMIC-PACIFIC PELECYPODA 137 



Subgenus MYOFORCEPS Fischer, 1886 



Type species by original designation, Lithodomus caudigerus Lamarck 

 = L. aristatus (Dillwyn). 



Surface of shell with a coating of lime which posteriorly is much 

 thickened and produced beyond the ends of the valves so as to form a pair 

 of twisted, alternating or crossed blades (like the blades of a pair of scissors) 

 and which serve to close-off the bilobate orifice of the burrow excavated in 

 rock or thick-walled shell. 



Llthopha&a (Myoforceps) aristata (Dillwyn) Plate 16, figures 2-2c 



Le Ropan, Adanson, 1757, Hist. Nat. Sin^gal Coquillag«s, p. 267, pi. 19, fig. 2. 

 Mytilus aristatus (Solander MS.), Dillwyn, 1817, Cat. Recent Shells., I, p. 303. 

 Modiola caudigera Lamarck, 1819, Anim. s. Vert., 6, p. 116 (after Encyclop. Meth., p. 



221, figs. 8a, 8b) — Philippi, 1846, Abbild. und Beschreib. Conchylien, bd. 2, 



p. 149, pi. 1, Modiola fig. 5. 

 Lithodomus caudigerus (Lamarck), Sowerby, 1824, Genera Shells, Lithodomus, fig. 4. 



— Reeve, 1857, Conch. Icon., vol. 10, Lithodomus, pi. 3, fig. 16 

 Lithophaga (Myoforceps) aristata (Dillwyn), Dall, 1898, Trans. Wagner Free Inst. 



Sci. Philadelphia, vol. 3, pt. 4, p. 800.— Dall, 1909, Proc. U. S. Nat. Museum, 



vol. 37, No. 1704, p. 153. — Hertlein and Strong, 1946, Zoologica, vol. 31, pt. 2, 



No. 5, p. 74. — Soot-Ryen, 1955, Allan Hancock Pacific Expedition, vol. 20, No. 



1, p. 98, pi. 10, figs. 53, 54. 

 Lithophagus calyculatus Carpenter, 1857, Cat. Mazatlan Shells, Brit. Mus., pp. 124, 125, 



No. 174. MS. drawing in U. S. National Museum. 



Shell small or of medium size (average length about 32 mm.), thin, 

 subcylindrical, modioliform, the beaks nearly terminal, the hinge line about 

 half the length of shell, the posterior half of the dorsal side descending, the 

 ventral margin straight or concave. Periostracum thin, light brown or 

 chestnut, covered by a thin, calcareous incrustation much thickened pos- 

 teriorly and produced beyond the end of the valves in the shape of two, 

 flattened blades which are straight or twisted so as to pass each other 

 alternately and resemble the blades of a pair of scissors. 



Length 36.7 mm.; height 10.5 mm.; diameter 9.5 mm. Manta, Ecuador, 



A borer into soft rock or into the thick wall of shells such as Chama, 

 Spondylus, and Ostrea. The opening of the burrow usually shows on the 

 surface as a small, bilobate slit which expands below into a large, bullet- 

 shaped cavity. 



Range — California to Peru. Also throughout the West Indies and 

 along the southeast coast of the United States; also West Africa. Panama: 

 Guanico; Pearl Islands. Canal Zone: Fort Amador. Ecuador: Esmeraldas; 

 Manta; Palmar; Santa Elena. Peru: Sechura (Dall). 



Subgenus DIBERUS Dall, 1898 



Type species by original designation, Lithophaga plumula Hanley. 



Shell with one or more radial sulci extending along the posterior um- 

 bonal slope with the calcareous encrustation covering the space between 

 them coarse and heavy, and generally developing a divaricate, plumelike 

 pattern. The calcareous prolongations at the end of each valve has the 

 shape of short wedges. 



