PANAMIC-PACIFIC PELECYPODA 123 



Septifer zeteki Hertlein and Strong, 1946, Zoologica, vol. 31, pt. 2, No. 5, p. 71, pi. 1, 

 figs. 1, 2. (Holotype from Taboga Island, Panama). 



Shells quite small, modloliform, widely expanded on the posterior side, 

 impressed along the ventral side and narrowed anteriorly, the valves there- 

 fore having an oblique trigonal form. Sculpture is formed by small radial 

 ribs which may branch at the ends, finely noded by concentrics, much 

 finer on the ventral slope. Color green, mottled with brown, the umbonal 

 angle carrying a row of large bristles. Margins continuously crenulate all 

 around, the beak with a small septum or decklike floor in the tip of the 

 umbonal cavity. 



Length 6.8 mm., greatest height 4.1 mm., diameter 3.5 mm. (Hertlein 

 and Strong). 



Range — Gulf of California to Peru and the Galapagos Islands. Ecuador: 

 Puerto Callo. Santa Elena. Panama: Taboga Island. (Hertlein and Strong). 

 Lagartillo. Peru: Bayovar, Sechura Bay. 



Genus MTTELLA Soot-Ryen, 1955 



Type species by original designation, Modiola guyanensis Lamarck. 



Shell elongate, modioliform, often thin, the beaks and umbones placed 

 near the anterior end. Posterior-dorsal margin straight, curved, or angu- 

 lated at the end, the ligament and its scar nearly as long. Resilifer long and 

 narrow, white, with a narrow pitted foundation. Anterior end relatively 

 short, its margin smooth. Anterior adductor scar distinct, small and of an 

 oblong, irregular outline, the posterior adductor confluent with the retractor 

 and forming a club or pipe-shaped impression. Valves convex or vaulted 

 along the umbonal slope, the dorsal-posterior side usually of a darker color, 

 often with a greenish shade. Periostracum thin, the umbones of the shell 

 commonly corroded. 



Three species. 



I. Shell broadly wedge-shaped, the ventral side often deeply impressed. 

 Ratio of length to height, about 7 to 4. 



M. guyanensis 



II. Shell longer, often razor-shaped and very thin. 



1. Shell exceedingly long, Solen-Vike and very thin. Length to height about 

 4 to 1. 



M. speciosa 



M. tumbezensis 



2. Shell shorter, the length to height about 2.4 to 1. 



Mytella guyanensis (Lamarck) Plate 12, figures 8, 8a 



Modiola guyanensis Lamarck, 1819, Anim. s. Vert., vol. 6, p. 112.— Delessert, 1841, Rec. 



Coq. decrites par Lamarck, pi. 13, fig. 9. 

 Modiolus guyanensis (Lamarck), Maxwell Smith, 1944, Panamic Marine Shells, p. S3, 



fig. 692H. 

 Modiola brasiliensis Reeve, 1857, Conch. Icon., vol. 10, Modiola, pi. 4, fig. 17 Guayaquil; 



pi. 6, fig. 31 Brazil. 



