304 AXEL A. OLSSON 



concentric cords which pass over them higher or more lamellose. The shell 

 is varied in its coloration, often a dull, violet brown, uniform or strongly- 

 mottled, sometimes with wide, broken rays of violet on a cream-yellow 

 base. The interior is white. 



A large, right valve in my collection from Santa Elena measures: 

 Length 55.2 mm., height 46.7 mm., diameter 16.5 mm. 



Range — Mexico to northern Peru. Panama: San Carlos; Biicaro; Burica 

 Peninsula. Ecuador: Pedernales; Charapota; Manta; Punta Blanca; Santa 

 Elena. Peru: Zorritos; Boca Pan; Mancora; Lobitos; Pacasmayo (Dall). 



Genus PROTOTHACA Dall, 1902 

 Type species by original designation, Venus dombeyi Lamarck ( = F. 

 thaca (Molina) d'Orbigny). 



Shell ovate and moderately convex, the surface marked with a can- 

 cellate sculpture, the radials usually the stronger. In the typical species, 

 the coloration is dull and the sculptural pattern is more or less clearly 

 divided into three areas; the anterior surface with both radial and scabrous 

 elements, the middle zone chiefly radial, the posterior with the concentrics 

 strongest. Lunule small, not impressed but sharply circumscribed; the 

 escutcheon is restricted to the left valve and consists merely of a narrow, 

 flat space, its margin rolled over so as to produce a slight overhang but not 

 sufficient to hide the ligament. Hinge with three cardinal teeth in each 

 valve, the middle left, and the middle and posterior right teeth being bifid; 

 the anterior right tooth is strong and high. Pallial sinus of medium size, 

 extending to about the middle. Valve margins crenulated except on the 

 posterior side. 



This group is mainly Pacific, the principal species living along the 

 west coast of South America, Japan, and in New Zealand. 



This genus may be divided into two subgenera as follows: 



L Shell large, with a dull or drab-colored surface and sculptured with 

 strong radial and concentric riblets, the concentric elements strongest 

 over the anterior slope. Cold water. 



Subgenus Prototkaca, s.s. 

 II. Shell smaller, with finer sculpture, the surface more highly colored, 

 often with varied pattern. Warm or tropical waters. 



Subgenus Tropithaca 



Protothaca (Protothaca) thaca (Molina) Plate 41, figure 1; 



Plate 53, figures 1, la 



Chama thaca Molina, 1782, Saggio Hist, de Chile, p. 178. 



Venus dombeyi Lamarck, Philippi, 1844, Abbild. und Beschreib. Conchylien, bd. 1, 



Venus, p. 127, pi. 2, Venus, fig. 1. 

 Paphia {Protothaca) thaca (Molina), Dall, 1909, Proc. U. S. Nat. Museum, vol. 37, No. 



1704, p. 269. 



Although Protothaca, s.s. is not a member of the tropical Panamic- 

 Pacific fauna, figures of its shell and hinge of P. thaca, its type species, are 

 given here in order that its characters can be better understood. P. thaca is 

 a species of the cooler waters of the Peruvian fauna and ranges from Lima 

 southward to Chile. 



