PANAMIC-PACIFIC PELECYPODA 125 



Lower California (Hertlein and Strong). Nicaragua: Bay of Fonseca. 

 Ecuador: Canoa; Atacames; Santa Elena. Peru: Tumbez; Zorritos; Punta 

 Picos; Negritos. 



3Iytella tnmbezensig (Pilsbry and Olsson) Plate 14, figure 7 



Modiolus (Modiolus) tumbezensis Pilsbry and Olsson, 1935, Nautilus, vol. 49, No. 1, pp. 



16, 17, pi. 1, fig. 5. 

 Mytella speciosa Soot-Ryen, 1955, Allan Hancock Pacific Expeditions, vol. 20, No. 1, 



pp. 52, S3, pi. 5, fig. 25; text-fig. 37 (not of Reev«, 1857). 



Shell about 40 to 45 mm. long, thin and much compressed. The beaks 

 are small. In a specimen 43 mm. long, the beak is about 6 mm. from the 

 end which is obliquely rounded. The posterior side is somewhat expanded, 

 its margin straight, rounded into the dorsal and ventral ones. The surface 

 is marked with small, regularly incised growth lines on the ventral slope, 

 much smoother on the posterior-dorsal side. Color of the shell is brown 

 or tan on the anterior-ventral side becoming green or olive minutely flecked 

 with chocolate on the posterior side. 



Length 41 to 44.5 mm.; height 18.5 to 21.5 mm.; diameter of a single 

 valve 4.25 to 4.5 mm. Tumbez, Peru. 



This is a small, thin, delicate species common along the tidal mud flats 

 of the river Tumbez in northern Peru. It is distinguished from M. speciosa 

 by its shorter and smaller shell. 



Range — Lower California to northern Peru. Mexico: Bahia de Magade- 

 lena (Soot-Ryen). Peru: Tumbez. 



Genus MODIOLUS Lamarck, 1799 

 Nomen conservandum. Opinion 325, International Com. Zool, Nomen. 



Type species by monotypy, Mytilus modiolus Linne. Recent, North 

 Atlantic. 



The name "Volsella" Scopoli, 1777, which has been in general use in 

 late years for this genus, has been suspended by a ruling of the International 

 Commission of Zoological Nomenclature (Opinion 325, dated Jan. 7th, 

 1955). 



Shell variable in size, ovate, oblong or obliquely elongated, the small 

 beaks adjacent and almost terminal, convex, the surface covered by a thin 

 or heavy, green, brown to nearly black periostracum, smooth or coarsely 

 pilose and often peeling off from dead shells. Surface under the perio- 

 stracum usually smooth except for growth lines. Dorsal margin curved, the 

 hinge edentulous except for a small projection near the anterior end of 

 ligament. Ligament subinternal, attached to a rather long, broad, curved, 

 inset resilifer. Interior of valves white to purple, the ventral margin with 

 a small byssal gap. Some species build a nest of pebbles and fragments of 

 shell held together by the threads of the byssus. 



Key to Panamic Modiolus 



I. Surface of shell is covered with a coarse, peeling periostracum, dark- 

 brown in color and carrying over its posterior portion a dense growth 

 of coarse bristles, the individual bristles being compound or serrated 



