PANAMIC-PACIFIC PELECYPODA 233 



sloping evenly to the ventral margin which is nearly straight. Anterior and 

 posterior umbonal slope rounded and prominent, their sides turned down 

 sharply and rather narrow. Hinge as illustrated, the laterals strong, the left 

 valve with a small, double pseudocardinal under the beak. Surface smooth 

 and polished, the growth lines fine, their distribution somewhat irregular. 

 Length 10.5 mm., height 8 mm., diameter 2.1 mm. (right valve). 



Zorritos, Peru, Holotype, ANSP 218883. 



This species is abundant in dredgings in Sechura Bay and at Zorritos. 

 It resembles B. mactroides (Conrad) from the Chesapeake Miocene of 

 Maryland and Virginia which is somewhat larger; B. dodona Dall from the 

 Chipola of Florida is also similar but smaller. 



Range — Peru. Peru: Sechura; Zorritos. 



Bornia ehlclaya, new species Plate 35, figure 13 



The shell is small, thin, rounded, trigonal, or high and hatchet-shaped, 

 subequilateral, the wide, flattened surface of the umbones submedian in 

 position, below, the surface of the disk is nearly flat and slopes slightly 

 towards the ventral margin which is wide and nearly straight. The beaks 

 are small, pointed a little forward and placed slightly in front of the middle 

 line. Color white or translucent, the surface smooth and glossy, usually the 

 growth lines show faintly, and often banded concentrically with narrow, 

 opaque and glassy ribbons. The right valve has a fairly strong hinge with the 

 lateral teeth and their sockets placed close to the center and on each side 

 of the excavated zone of the ligament. In the left valve, the hinge is weaker 

 but has a large, thin, anterior lateral tooth and behind it a small, pseudo- 

 cardinal. Pallial line is wide and entire. 



Length 7.1 mm., height 5.9 mm., diameter 1.1 mm. (a right valve). 

 Chimbote, Peru. Holotype, ANSP 218890. 



In shape, similar to a triangle with rounded basal corners. It is a 

 larger, higher, and more depressed species than B. zorritensis. 

 Range — Peru. Peru: Chimbote; Negritos. 



Family MONTACUTIDAE 



Shell generally small, ovate to oblong, thin or heavy, depressed or con- 

 vex, the anterior side the longer. Ligament mostly internal, subumbonal or 

 lodged in an elongated, oblique resilifer in the hinge margin behind the 

 beaks. Surface smooth or with concentric growth lines and sometimes radial 

 undulations. Habit of most species appears to be commensal. 



Genus MTSELLA Angas, 1877 



(Rochefortia Velain, 1877, type species, R. australis Velain.) 



Type species by monotypy, M. anomala Angas. Australian waters. 



Shell small, ovate, subquadrate to rounded-trigonal, the surface gen- 

 erally depressed, the anterior side longer. The hinge plate is stout and 

 bears a short, central, subumbonal resilifer; in the right valve, the resilifer 

 is bordered by two small teeth, one on each side, the anterior tooth is the 

 larger, and above each of these there is a grooved socket in which the 

 thickened margin of the left valve is inserted. The pallial line is a plain, wide 

 band or ribbon connecting the adductor scars of which the anterior scar is 

 the larger. Species live free or even up in the burrows of Crustacea and 

 other marine invertebrates. 



