PANAMIC-PACIFIC PELECYPODA 129 



Anterior reflex margin ends rather sharply a short distance behind the beaks, 

 usually with a few small dysodont teeth along it and which grade into 

 the normal crenulations forward. Anterior adductor scar large, ovate, the 

 posterior indistinct in my specimens. Borers. 



Mnscnlus (Gregariella) coarctatus (Carpenter) Plate 16, figures 4-4d 



Crenella coarctata Dunker, Carpenter, 1855, Cat. Mazatlan Shells, Brit. Mus. p. 123, 



No. 172. 

 Lithodomus coarctata Dunker, Reeve, 1857, Conch. Icon., vol. 10, Lithodomus, pi. 3, 



fig. 14. 

 Lithophaga coarctata Dunker, 1883, Conch. Cab., 2d ed., pp. 28, 29, pi. 5, figs. 2, 3. 

 Modiolaria (Gregariella) coarctata (Dunker), Lamy, 1937, Jour, de Conchyl., vol. 81, 



p. 42. 

 Gregariella coarctata (Carpenter), Soot-Ryen, 1955, Allan Hancock Pacific Expeditions, 



vol. 20, No. 1, pp. 77, 78, pi. 9, fig. 48 ; text-fig. 64. 



Average size about 27 mm., elongately modioliform, with a wide um- 

 bone weakly sulcated across the middle. The small coiled beak is placed 

 near the anterior end, the posterior end of the valve narrowly rounded 

 to pointed, tufted with periostracal hairs and flexed slightly towards the 

 right. Periostracum thin, brown over a thin pearly layer. Surface sculpture 

 is formed by fine threadlets radiating from the beak and divaricated 

 along the keeled umbonal slope, except on the anterior-middle zone which 

 is smooth. Inner margins of the valve minutely crenulate except in the 

 mid-zone. Hairs of the periostracum rather sparse and confined to the 

 umbonal keel, forming a tuft at the end. 



Length 28 mm.; height 11 mm.; diameter 10.8 mm. 



Manta, Ecuador. 



This is a true rock borer forming smooth-walled, bullet-like holes. 

 It is fairly common at Manta associated with Lithophagas. Resembles 

 M. coralliophaga Gmelin of the West Indies but is more slender and less 

 pointed at the posterior end. 



Range — Lower California to Ecuador. Panama: Taboga Island (Soot- 

 Ryen). Ecuador: Manta. 



Mnscnlus (Gregariella) ehenal (Recluz) 



Mytilus chenui Recluz, 1842, Revue Zool., vol. 5, p. 306. 



Gregariella opifex Dall, 1921, Bull. 112, U. S. Nat. Museum, p. 22. (Probably not of 



Say, 1825 fidi Soot-Ryen). 

 Gregariella chenui (Recluz), Soot-Ryen, 1955, Allan Hancock Pacific Expeditions, vol. 



20, No. 1, p. 78, pi. 8, fig. 40 ; text-fig. 65. 



This is typically an East American species but has been recorded from 

 the Pacific Coast by Dall and Soot-Ryen. I have seen no specimens from the 

 Panamic zone. According to Soot-Ryen, the Hancock collections contain 

 many samples of dead specimens taken in depth ranging from 16 to 50 

 fathoms. 



Range — Southern California to the Bahia de la Independencia, Peru. 

 (Soot-Ryen). 



Genus CBENELLA Brown, 1827 



Type species by monotypy, Mytilus decussatus Montagu. Recent, 

 North Atlantic. 



