130 AXEL A. OLSSON 



Shell small, often minute, obliquely ovate, convex, with small, incurved, 

 prosogyrate beaks. Prodissoconch often well preserved, smooth. Interior 

 pearly, the outer surface covered when fresh by a thin, closely adhering 

 periostracum. Hinge mytiloid, with small toothhke knobs on the anterior 

 margin bordered on the posterior side by a deep, furrow-like pit for the 

 attachment of the ligament which is wholly internal. Surface sculpture 

 composed of fine radials crossed and decussated by concentrics, often 

 divaricated along the middle line of the umbonal slope. Inner margin finely 

 crenulated by the ends of the external riblets all around, sometimes ex- 

 tending into the hinge itself to produce a pseudotaxodont pattern there. 



Three species appear regional. 



Key to species of Panamic-Peruvian species of Crenella 



I. Shell large, height 25 mm. 



Crenella megas. Panama Bay in 33 fathoms. 



II. Shell small, minute, height not over 4 mm. 



1. Valves nearly oval in shape, the dorsal and ventral sides nearly alike. 



Crenella ecuadoriana 



2. Shell subcircular to rhombic, the dorsal and ventral sides unlike. Pos- 

 terior-dorsal margin straight. 



Crenella caudiva, new species. 



Crenella ecuadoriana Pilsbry and Olsson Plate 17, figures 7, 7a 



Crenella ecuadoriana Pilsbry and Olsson, 1941, Proc. Acad. Nat. Sci. Piiiladelphia, vol. 



93, pp. 55, 56, pi. 18, figs. 2, 3. 

 Crenella divaricata d'Orbigny, Hertkin and Strong, 194-6, Zoolcgica, vol. 31, pi. 2, 



No. 5, pp. 75, 76, pi. 1, figs. 12, 13 (in whole or part). 



Shell small, the average length about 3.25 mm., plump, elongately 

 oval, nearly symmetrical, a line through the middle of the umbonal slope 

 divides the surface into nearly equal halves. Surface sculpture consists of 

 small, weakly decussated nblets which increase along the umbonal slope 

 and near the ventral margin by occasionally forking and by the addition 

 of intercalaries between their branches. In addition, the sculpture is divided 

 into three contrasting areas by straight lines extending from the beaks to 

 the margins and against which the small, simple (not forking) riblets abut 

 sharply. 



This small species is locally plentiful in beach drift at Santa Elena. 

 As fossil, it occurs in the Pliocene of Panama and Ecuador. Some authors 

 have considered this small Crenella as the same as C. divaricata d'Orbigny 

 described from Cuba. The Caribbean-West Indian Crenellas have not been 

 thoroughly worked over but a comparison of C. ecuadoriana with the speci- 

 mens from Bocas del Toro, Panama, show that the Caribbean shell is 

 thinner, less symmetrical in shape, usually with a larger, more conspicuous 

 prodissoconch and with weaker sculpture. 



Range — Lower California to northern Peru. 



Crenella eaudlTa, new species Plate 17, figure 2 



The shell is small, moderately convex but not plump, unsymmetrical, 

 the shape is irregularly subrhomboidal, the posterior side higher than the 



