132 AXEL A. OLSSON 



Genus ADULA H. and A. Adams, 1857 



Type species by original designation, Mytilus soleniformis d'Orbigny. 



Rock borers. Shell elongated with closed valves, subcylindrical but 

 generally curved, the ventral margin being broadly impressed along the 

 midzone. The beaks small, flattened and placed slightly in front of the 

 middle and usually so deeply corroded that the pearly inner layer is ex- 

 posed. Posterior end of valves a little wider, with an umbonal angulation 

 generally covered by a thick crust of agglutinated grains of sand form- 

 ing a plug or subtruncated stump. Small shells have the shape of a straight 

 Modiolus with a sharply angled umbonal ridge more than half the length 

 of the shell and the surface sculptured with fine, radial riblets; the perio- 

 stracum thin and of a brown or golden-yellow color. Hinge edentulous, the 

 ligament narrowly linear and subinternal. Inner margins of the valves 

 finely crenulated except along a part of the ventral side. Inner layer of 

 shell pearly, the outer layer thin, and covered by a brown periostracum 

 which is usually retained only along the ventral side and under the cover 

 of agglutinated sand grains at the posterior end. 



The typical species of Adula are rock borers. A large colony of A. 

 soleniformis panamensis was found near Las Tablas, Panama, boring into 

 a volcanic, tuffaceous agglomerate, mostly along the softer layers. The 

 bores are small, deep shafts in which the small clam fits snugly when re- 

 tracted, closing off the entrance by its bilobed stump of agglutinated sand 

 grains covering the posterior end. The umbones of these shells are deeply 

 worn. 



Adnia soleniformis (d'Orbigny) Plate 16, figure 3b 



Mytilus soleniformis d'Orbigny, 1846, Voy, Am^r. M6tid., vol. 5, p. 649, pi. 85, figs. 



17, 18 Paita, Peru. 

 Adula soleniformis (d'Orbigny), Dunker, 1882, Syst. Conch, Cab., vol. 8, p. 24, tab. 6, 



figs. 4, 5. — Soot-Ryen, 1955, Allan Hancock Pacific Expeditions, vol. 20, No. 



1, p. 90. 



Typical specimens of A. soleniformis from northern Peru and Ecuador 

 are longer and narrower than Panama examples and also attain a larger 

 size (Manta, length 39.5 mm.). The largest Panama shell has a length of 

 28 mm., and the valves are shorter and higher posteriorly; these Panama 

 shells are separated as a subspecies -panamensis. (PI. 16, figs. 3, 3a) 

 Small shells of A. -panamensis resemble a small Musculus and have a strong 

 umbonal ridge, the ventral side is deeply impressed and the whole surface 

 is covered with fine radial riblets. The posterior ridge bears several long 

 algal-like bristles, but its surface is soon covered with a mass of agglutinated 

 grains of sand which cannot be removed without damage to the shell under- 

 neath. The bores of A. panamensis consist of small shafts, usually closely 

 crowded together, smaller at the lower end, often appearing more or less 

 bilobate at the entrance which is closed off by the stumplike mass of 

 agglutinated sand grains covering the posterior end of the valves. Adula 

 is evidently not rare at localities favorable to its growth. 



Range — Panama to northern Peru. Panama: Lagartillo near Las Tablas; 

 Guanico. Ecuador: Manta. Peru: Mancora; Paita. 



