448 AXEL A. OLSSON 



and deeply lobate mesoplax. The figured specimen was obtained at Jaramijo 

 along with more abundant P. acuminata. Its measurements are: length 

 32 mm., height 17.8 mm., greater diameter 17.5 mm. 



Range — Mexico to Ecuador. Mexico: Guaymas; Mazatlan (Turner). 

 Panama: Perico Island (Cuming and Sowerby). Ecuador: Manta (Turner); 

 Jaramijo. 



Genus HASTASIA Gray, 1851 



Type species by subsequent designation, Stoliczka, 1870, Pholas 

 melanura Sowerby. 



The shell is broadly pholadiform, each valve with a single median sulcus 

 dividing the surface into two unequal parts; the anterior section with 

 scabrous sculpture and beaked at the end; the posterior section with smooth 

 concentrics. The anterior side is widely open in the young, later closed by 

 the large, paired callum, extensions of which are prolonged upward along 

 the beaks, and downward along the ventral side and partly around the 

 posterior end. There is no protoplax as the function of this plate is per- 

 formed by the extension of the paired callum. The mesoplax in the juvenile 

 stage is a small, flat, semicircular plate, and in one piece. The siphonoplax 

 is chitinous. An incipient metaplax and hypoplax is present or absent. The 

 internal apophyses are long and slender. 



This group has generally been considered as a subgenus of Pholadidea, 

 but its characters are sufficiently distinct to warrant full generic status. 



Hastasia melannra (Sowerby) Plate 78, figure 6; 



Plate 79, figure 1 



Pholas melanura Sowerby, 1834, Proc. Zool. Soc. London, pp. 70, 71. — Sowerby, 18+9, 



Thes. Conch., vol. 2, p. 499, pi. 107, figs. 78, 79. 

 Pholadidea melanura (Sowerby) Carpenter, 1857, Cat. Mazatlan Shell, Brit. Mus., p. 



8, No. IS. — Maxwell Smith, 1944, Panamic Marine Shells, p. 69, fig. 874. 



Some free valves from Guanico are large and measure over 2/4 inches 

 in length; still other larger valves in the collection are from San Pedro, a 

 place to the south of Manglaralto in Ecuador. The shell is heavy. The 

 median sulcus is deep, cross-threaded, and shows well in the interior as a 

 coarse rib. In the adult, the anterior gap is closed by a large, convex paired 

 callum extensions of which are carried upward to the point just behind 

 the beaks, and also along the ventral margins, and in these places, it re- 

 places the protoplax and the hypoplax. 



Range — Lower California to Ecuador. Mexico: Mazatlan. Panama: 

 Guanico; Burica Peninsula. Ecuador: Manta; San Pedro. 



Hastas;a tubifera (Sowerby) Plate 79, figures 4-4d 



Pholas tubifera Sowerby, 1834, Proc. Zool. Soc. London, p. 71. (Sinum Caraccensem 

 Bahia de Caraques).— Sowerby, 1849, Thes. Conch., vol. 2, Pholas. p. 499, pi. 

 106, figs. 64, 65. 



Pholadidea tubifera (Sowerby), Gall, 1909, Proc. U.S. Nat. Museum, vol. 37, No. 1704, 

 p. 277.— Maxwell Smith, 1944, Panamic Marine Shells, p. 69, fig. 877. 



This is a relatively small species probably never exceeding 40 mm. in 

 length. In this species, the protoplax remains in connection with the callum 

 but joined with it only by a narrow constriction or neck. Specimens which 

 have lost the protoplax show the area beneath it to be covered by the wide, 

 flattened, appressed surface of the reflected margin of the valves thus 



