114 AXEL A. OLSSON 



Shell narrowly elongated, decidely arcuate, the base being strongly in- 

 flected and the dorsal margin arched. The ventral side is flattened, thereby 

 emphasizing the angled edge of the umbonal slope. Color an olive-brown, 

 the periostracum thin, smooth, often worn-off in spots, the surface beneath 

 with strong concentrics. The small beaks are almost terminal, the short 

 anterior margin below folded so as to form 3 or more weak teeth and shallow 

 pits. Resilifer is nearly as long as the arched dorsal margin, narrowly hnear, 

 white, its basal edge strongly pitted. Posterior adductor scar pipe-shaped, 

 the upper arm long and narrow, confluent with the retractor scar at its 

 end. The anterior adductor scar present. 



This recognizable species was referred to Mytella and placed in the 

 synonym of M. falcata (d'Orbigny) by Soot-Ryen but its relations lie 

 closely with Mytilus. It is apparently a mud-flat species, especially common 

 along the mangrove-fringed shores of the Gulf of Guayaquil. 



Range — Salvador to the Gulf of Guayaquil. Salvador: Gulf of Fonseca 

 (Hertlein and Strong). Nicaragua; Monypenny Point (Hertlein and 

 Strong). Ecuador: Huaquilla (Dall); Posorja; Puna Island; Santa Elena. 



Genus SEMIMYTILUS Soot-Ryen, 1955 



Type species by original designation, Mytilus algosus Gould, 1850. 



Shell small or of medium size, mytiliform, the beaks almost terminal, 

 the dorsal margin almost straight with the ligament along it narrow and 

 linear. Anterior end short, reflexed below the beak, and extended forward 

 forming a short, slightly pouting extremity, its rolled over margin smooth 

 or rarely weakly denticulated, its external surface forming a small lunule 

 circumscribed by a line. Anterior adductor scar very small, continued into 

 the irregular pallial line which follows the ventral margin closely, the pos- 

 terior adductor and retractor joined forming an irregular, pipe-shaped scar. 

 Surface smooth or with undulated concentrics, the periostracum thin, brown 

 or black, shiny or with a silky sheen. 



Two species. 



Key to species of Semimytilus 



1. Length about 45 mm; anterior end quite narrow, the posterior narrow 

 to medium well rounded; pallial and muscle impression usually distinct. 

 Peruvian faunal zone. 



S. algosus 



2. Shell smaller and more stubby, the posterior end more widely rounded; 

 pallial and muscle impression weakly marked. Paita Buffer Zone. 



S. nonuranus 



SemlmjiJlus algosns (Gould) Plate 14, figure 8 



Mytilus algosus Gould, 1950, Proc Boston Soc. Nat. History, vol. 3, p. 344.— Gould, 

 1852, United States Exploring Expedition, vol. 12, p. 450, atlas, 1856, pi. 41, 

 figs. 566, 566a. 



Semimytilus algosus (Gould), Soot-Ryen, 1955, Allan Hancock Pacific Expeditions, vol. 

 20, No. 1, pp. 25, 26, pi. 4, fig. 17; text-figures 8, 9, 14, 15, 16. 



Mytilus dactyliformis Hup6, 1854, Gay, Hist. Chile, Zool., vol. 8, p. 310, pi. 5, fig. 6. 



