440 AXEL A. OLSSON 



and projects beyond the tip of the beaks but slightly. Myophoral plate 

 relatively large, forming a shelflike lamina attached below and within the 

 hinge margin and extending into the umbonal cavity, its inner edge open 

 and free. Surface with concentric sculpture. Adductor scars of unequal size, 

 the pallial sinus narrow and deep, extending into the inner sections of the 

 shell cavity. 



Gastroctaaena (Gastrochaena) tmncata Sowerby 



Gastrochaena truncata Sowerby, 1834, Proc. Zool. Soc. London, p. 21, Hab. Found in 

 Spondyli, in Sinu Panamensi (Isle of Perico). — Sowerby, 1878, Conch. Icon., 

 vol. 20, Gastrochaena, pi. 3, fig. 19. — Sowerby, 1884, Thes. Conch., vol. 5, p. 

 130, No. 23, pi. 470, fig. 13. 



Shell large (length 36 mm.), elongate, and narrowly modioliform, 

 convex, the valves in contact only along the dorsal and posterior margin, 

 the posterior end somewhat truncate or widely rounded. Ventral gap large, 

 elliptical, and extending across nearly the whole length of the ventral 

 margin. Shell texture relatively solid and sculptured with coarse, wrinkled 

 growth incrementals, the surface covered by a brown epidermis, which is 

 heaviest on the posterior portion. An internal myophore present. 



This is a large species, broad, square or widely subtruncate at the 

 posterior end, and much narrower and pointed at the anterior end. 

 Range — Mazatlan to Panama. Panama: Panama. 



Gastrochaena (Gastrochaena) ecnadoriensis, new species Plate 80, figures 8, 8a 



Valves relatively large, reaching a length of about 25 mm., thin, 

 obliquely modioliform, the ventral gap large, extending the whole length 

 of the ventral margin from the anterior extremity to the posterior-ventral 

 corner. The small beaks are nearly terminal, the anterior margin, projecting 

 beyond them, short. Dorsal margin straight, the external ligament extending 

 about halfway along it, the posterior end of the valves wide, the margin 

 itself widely rounded to subtruncate. There is a weak umbonal ridge running 

 obliquely across the disk of the valve from the beak to the posterior-ventral 

 edge along which the concentric ridges and emphasized growth wrinkles are 

 bent backwards almost at right angles. The myophoral shelf within the 

 umbonal cavity large. Pallial sinus long and narrow, extending deep into 

 the cavity of the shell. Adductor scars of unequal size. 



Length 25 mm., height 12 mm., diameter of both valves 11 mm. 



Taken from a thick shell Spondylus. Punta Montanita above Manglar- 

 alto, Ecuador. Holotype, ANSP 218914. 



Subgenus ROCELLARIA Blainville, 1828 

 Type species by monotypy, Gastrochaena modiolina Lamarck = Mya 

 dubia Pennant. Europe. 



Shell small or of medium size, thin, moderately convex, the valves 

 obliquely modioliform, with the umbones and beaks strongly anterior, the 

 anterior end pointed. Ventral margin is widely open or gaping, the valves 

 in contact only along the dorsal margin and at the ends. No internal myo- 

 phoral plate. Surface sculptured by strong, concentric wrinkles, and accen- 

 tuated growth lines, sometimes becoming lamellar. Pallial sinus moderately 

 deep and sharply angular. 



