400 AXEL A. OLSSON 



Range — Panama to northern Peru. Panama: Bahia Honda, El Largar- 

 tillo near Las Tablas; Isla del Rey, Pearl Islands (Tomlin). Peru: Tumbez. 



Tellinidella mompichensis, new species Plate 72, figure 3 



Shell thin, narrowly elliptical, the length more than twice the height, 

 the beaks median, the two sides nearly alike. Color rose or white, often 

 more or less banded. Rostral area narrow but well marked, deeply excavated, 

 sculptured with cross threads, without radial striations. Like T. purpureus 

 in general form but longer and with finer surface reticulation. 



Length 57 mm., height 27.3 mm., diameter 5 mm. (right valve). Mom- 

 piche, Ecuador. Holotype, ANSP 218947. 



My three right valves from Mompiche are of a light pink color, per- 

 haps in part due to beach exposure. An entirely white shell was obtained 

 in dredgings at Zorritos. 



Range — Ecuador and Peru. Ecuador: Mompiche. Peru: Zorritos. 



Genus MOERELLA Fischer, 1877 

 Type species by monotypy, Tellina donacina Linne. Recent, European 

 seas. 



Shell generally small, subquadrate to subelliptical, depressed or slightly 

 convex, the posterior side much shorter, its dorsal margin descending rather 

 sharply, sometimes a little flexed, and often subtruncated at the 

 end. Surface smooth or marked with concentric growth lines and in 

 the subgenus Scissula by incised lines cutting across the concentrics diagonal- 

 ly. Hinge as in Eurytellina, the right, anterior lateral tooth being placed 

 close to the cardinal; the lateral teeth in the left valve are small and hardly 

 represent more than a thickening of the valve margin so as to fit between 

 the grooved sockets of the opposite valve. 



Three subgenera may be recognized in the Panamic area. 

 L Shell usually small (seldom above 25 mm.), subquadrate to subelliptical 



in shape. 

 L Surface plain or marked only with growth line concentrics. 



Subgenus Moerella, s.s. 



2. With incised lines cutting diagonally across the concentrics. 



Subgenus Scissula 

 H. Shell large (40 mm. or more), subovate to subelliptical, the anterior 

 side expanded, convex; the posterior side shorter, pointed at end and 

 somewhat twisted. 



3. Shell white, often with the umbones colored rose-pink. 



Subgenus Elpidollina 



Key to species of Panamic-Pacific Moerella, s.s. 

 Surface marked with concentric sculpture only, sometimes reduced to 

 fine or hairlike growth lines or with stronger, incised lines. Posterior flexure 

 weak. 



A. Shell subtrigonal in shape, the beaks subcentral. 



1. Moderately convex and of medium weight, the posterior side short, 



flexure weak. Surface marked with fine, raised, hairlike concentrics. 



Color white. 



M. meropsis 



