236 AXEL A. OLSSON 



2. Shell smaller (length 8.5 mm.), the anterior side not twice the length 

 of the posterior, its end more evenly rounded; sagrination fine. 



0. zorrita 



B. Shell depressed or slightly convex. 



3. Anterior-dorsal margin descending, hence not parallel with the ventral 

 side, 



0. peruviana 



II. Surface without evident sagrination. 



C. Mid-zone of the valves impressed, the ventral margin, therefore, strongly 

 inflected or sinuated. 



4. Length of shell between 8 and 12 mm. 



O. margarita 



D. Mid-zone of valves not flattened or impressed. 



5. Valves with an oblique axis, the posterior-dorsal side descending sharply. 

 Surface generally chalky and marked with coarse concentrics. 



0. sechura 



6. Valves, oblong, boat-shaped, convex. Sculpture formed by fine, irregular 

 concentrics. 



0. jipijapa 



Orobitella zorrita, new species Plate 35, figure 3 



The shell is of medium size (length 8.4 mm.), oblong, boat-shaped, 

 convex, subequilateral, the anterior side a trifle longer, its dorsal margin 

 at first straight and parallel with the basal or ventral side but shortly 

 passing into the curve which forms the widely rounded, anterior end. The 

 posterior side is shorter, its dorsal margin descending from the beginning, 

 its end more narrowly rounded. The general surface is smooth except for 

 fine, incremental growth lines, its color white or finely banded at 

 the ends, the effect produced by alternating ribbons of opaque and glassy 

 white; at the ends, there are traces of a thin, light brown periostracum which 

 once covered the whole disk. The microscopic sagrination is extremely fine 

 and faint, visible only over the sides and under high magnification. The 

 inflation of the valves is greatest over the anterior-umbonal slope, the sur- 

 face of the disk being a little impressed across the mid-zone. The ventral 

 margins straight and plain. 



Length 8.4 mm., height 6 mm., diameter 1.8 mm. Zorritos, Peru. Holo- 

 type, ANSP 218886. 



This species resembles 0. stearnsii Dall but in the Peruvian shell, the 

 posterior side is longer and the anterior end has a more even, semicircular 

 curve. 



Range — Peru. Peru: Zorritos. 



Orobitella steams! (Dall) Plate 36, figure 2 



Sportella stearnsi Dall, 1899, Proc. U. S. Nat. Museum, vol. 21, No. 1177, p. 885, pi. 87, 

 figs. 9, 12.— Hertlein and Strong, 1947, Zoologica, vol. 31, pt. 4, pp. 137, 138. 



Shell of moderate size for the genus, inequilateral, not very convex, 

 White, with an almost imperceptible yellowish epidermis; anterior dorsal 

 margin nearly straight, the base parallel with it, the ends bluntly rounded; 

 surface nearly smooth, with faint incremental lines and microscopic sagrin- 

 ation; teeth normal, strong, the posterior cardinal prominent, vertical; liga- 

 ment strong, external, on a nymph; resilium well developed, its area of 

 attachment thickened; posterior adductor scar rounded, unusually large. 

 Lon. 13.5, alt. 10, diam. 5 mm. 



