PANAMIC-PACIFIC PELECYPODA S31 



Mactra (Mulinia) bistrigata Morch, 1860, Malak. Blatter, bd. 7, p. 182 "Realejo, 



Nicaragua". 

 Mactra bistrigata Weinkauff, 1884, Conchy.-Cab. von Martini-Chemnitz, bd. 11, abt. 



2, Mactracea, p. 102, taf. 3+, figs. 5-7 "Busen von Panama (Oersted)". 

 Mulinia pallida (Broderip and Sowerby), Dall, 1909, Proc. U.S. Nat. Museum, vol. 37, 



No. 1704, p. 274.— Lamy, 1918, Jour, de Conchy!., vol. 63, No. 4, p. 335.— 



Hertlein and Strong, 1950, Zoologica, vol. 35, pt. 4, p. 233. 

 Mulinia bradleyi Dall, 1894, Nautilus, vol. 8, p. 6, pi, 1, right figure. Panama, Bradley. 



USNM. 



Average size about 35 mm., but occasional specimens may attain a 

 length of nearly 65 mm. The valves are trigonal in shape, with nearly 

 median beaks and umbones, ventricose, the two ends subequal, the anterior 

 margin rounded, the posterior descending to a pointed or truncated end. 

 Posterior-dorsal area wide, flattened to depressed, limited externally by an 

 angled line. Surface smooth, often polished or marked only by fine lines of 

 growth, white or cream-colored, and covered by a thin, straw-colored perio- 

 stracum, heaviest on the posterior-dorsal area. Hinge stout, with a deep 

 chondrophore and strong, equidistant, lateral teeth. The ligament as typical 

 of the genus is entirely internal, the scar of the tensilium showing as a 

 scar in the roof of the chondrophore. 



Length 63.7 mm., height 53.4 mm., diameter 33 mm. Sua, Ecuador. 



Length 52.2 mm., height 40.3 mm., diameter 26.4 mm. Old Panama, 

 Panama. 



This is the common mactrid in the southern portion of the Panamic- 

 Pacific Province, somewhat variable in shape and in the thickness of its 

 valves. Typically, it has high, prominent umbones, its posterior end some- 

 what narrowed and produced. The surface is usually polished over the 

 umbones and midzone with scattered remnants of the thin, cream-gray 

 periostracum remaining near the ventral margin. M. bradleyi Dall, 1894, 

 described from Panama, is a questionable form with a more squared off 

 outline and a fringed, persistent periostracum. Young specimens of M. 

 pallida, especially if dredged, frequently show a strongly fringed perio- 

 stracum which may occasionally persist until maturity. M. modesta Dall, 

 1894 (A/, coloradoensis Dall, 1894) is a northern, estuarine form with a 

 coarse heavy shell. 



Range — Gulf of California to northern Peru. Panama: Burica Penin- 

 sula; Guanico; Biicaro; San Carlos; Garachine; Old Panama. Panama Canal 

 Zone: Venado Beach. Colombia: Isla del Gallo. Ecuador: Esmeraldas; Sua; 

 Cojimenes; San Francisco; Canoa; Manta; Santa Elena. Peru: Tumbez; 

 Zorritos; Punta Picos; Mancora; Negritos; Palta. 



Subfamily PTEKOPSEDINAE 



Shell ovate, the anterior-umbonal region inflated, the posterior side 

 low and depressed. Posterior-dorsal area absent or but weakly defined. 

 Substance of shell thin and delicate. Hinge with the lateral teeth reduced 

 or partly obsolete. Siphons wholly retractible. 



Genus RAETA Gray, 1853 

 Type species by monotypy, R. campechensis Gray [=.R. plicatella 

 (Lamarck), zzzR. canaliculata (Say)]. 



The shell is fairly large, thin, the anterior side high, rounded and 



