PANAMIC-PACIFIC PELECYPODA 219 



Length 20.2 mm.; height 20.6 mm.; diameter 6.4 mm. left valve, Manta, 

 Ecuador. 



First described from the Galapagos Island, this species is also common 

 along the coast of Peru northward to Panama. Some specimens are similar 

 to the Caribbean C, orbiculata, but in general, the Pacific shell has a coarser 

 and cruder sculpture, with a greater tendency for the ribs to split ventrally. 



Range — Peru northward to Panama and the Galapagos Islands. Pan- 

 ama: Bucaro. Ecuador: Manta; Puerto Callo; Santa Elena. Peru: Zorritos; 

 Boca Pan; Mancora. 



Genus DIVARICELLA von Martens, 1880 



Type species by monotypy, Lucina angvlijera von Martens (=Lucina 

 ornata Reeve). Recent, Mauritius. 



Shell subcircular or rounded, plump or convex, equivalve, and sub- 

 equilateral, white or glassy. Umbones full, median but with the beaks 

 prosogyrate and always placed a little closer to the anterior side. No dorsal 

 areas but a small lunule, larger in the right valve is present. A minute or 

 small nepionic shell, sculptured with fine, nondivaricating, concentric 

 threads, caps the beak. Hinge lucinoid, with the teeth variably developed, 

 often partly obsolete; when fully developed with a single strong cardinal 

 tooth in the right valve bordered by a socket on each side; the left valve 

 has two smaller cardinal teeth; laterals are variable, often strong, the an- 

 terior lateral placed close to the cardinals, the posterior one more distant. 

 Surface sculpture is highly characteristic; it is formed by concentric incised 

 lines at evenly spaced intervals, which begin at the lateral margins and 

 swing obliquely upwards crossing the growth lines and heavier resting marks 

 to meet along the anterior side of the umbonal slope where they form a line 

 of divaricating angles pointing dorsally; these lines form between them flat, 

 ribbon-like bands, their edges face dorsally as if made up of plates over- 

 lapping each other shingle-fashion. Shell cavity deep, the adductor scars 

 lucinoid, subequal, and with the pallial line following quite close to the 

 ventral margin which is finely crenulated. 



Two or three species of Divaricella usually occur in each faunal area 

 but because of their close resemblance, identification is at times difficult. 



The following key may be helpful in the determination of the Panamic- 

 Pacific species. 



1. Nepionic shell relatively large and sculptured with fine, threadlike 

 concentrics. Apex of the angles along the line of divarication rounded 

 or obtuse, often partly obsolete or smooth. Hinge well developed, the 

 lateral teeth persistent. Ventral margins generally crenulated. 



P. ferparvula 



2. Nepionic shell small, hardly distinguishable. Apex of angles of divarica- 

 tion sharp and acute. Hinge weaker, the lateral teeth subobsolete. 

 Ventral margins mostly plain. 



D, eburnea 



