386 AXEL A. OLSSON 



Bb. Ornamentation of the posterior-dorsal area more complex, the sulci 

 showing one or more lines of zigzag bends. 



Subgenus Pisostrigilla, new subgenus 



General key to Panamic-Pacific species of Strigilla 

 I. Surface ornamentation fairly simple, the incised lines strongly oblique 

 to the margin but not flexed or divaricated at any point. 



Subgenus Simplistrigilla 



1. Sulci rather coarse, run obliquely across the disk from the ventral to 

 the dorsal margin. Color white. 



S. strata 

 II, Surface ornamentation more complex, the sulci flexed or divaricated 



along one or more lines of sharp, angular bends. 

 A. Ornamentation of the posterior-dorsal area simple, its sculpture pro- 

 duced by straight, tangential sulci, often fine and crowded, or if more 

 irregular, do not develop a fixed pattern. 



Subgenus Strigilla, s.s. 



Aa. With two lines of sharp divarication, the principal one along the pos- 



terior-umbonal slope, a smaller one along the anterior-umbonal slope. 



a. Shell large, its length between 25 and 45 mm. 



2. Valves subcircular, depressed, white (rarely shaded with pink), the 

 lines of sulci fine. 



S. disjuncta 



b. Of average smaller size, the sulci coarser. Color white or rose-red. 



ba. Shell of medium size, length between 15 and 25 mm. 



3. Ornamentation along the posterior-dorsal area produced by crowded 

 tangential lines only. Color white or red. 



S. carnaria 



4. Ornamentation of the posterior-dorsal area more irregular, the sulci 

 sometimes irregularly flexed at the margin. 



S. chroma 



bb. Shell much smaller, its length usually less than 10 mm. 



5. Valves pea-shaped, white, often with pink umbones. Sulci along the 

 middle zone stand almost vertical to the ventral margin. The anterior- 

 umbonal flexure often partly effaced by a smooth tract. 



S. cicerula 



6. Similar to last, generally plain white, the middle sulci more inclined, 

 the anterior line of divarication weaker but without a smooth tract, 

 the anterior sulci often irregular or branching. 



S. dichotoma 

 Ab. With only a single line of sharp flexure (posterior or main line of 

 divarication). 



7. Shell small, convex, subequilateral, the beaks nearly median. The 

 sulci are fine and regular. 



S. ervilia 

 B. The posterior-dorsal area has a middle line of strong flexing of the 

 sulci, the lines along it showing sharp, zigzag bends. 



Subgenus Pisostrigilla 



8. Shell relatively small (6 to 7 mm., in length), convex, subcircular, 

 white in color. 



S. panamensis 



Subgenus STRIGILLA, s.a. 

 (Rombergia Dall, 1900.) 

 Shell of medium or large size, suborbicular and slightly inequilateral, 



