196 AXEL A. OLSSON 



ing the hinge Hne and placed near the anterior one-third, surface covered 

 with a dark brown to nearly black, closely concentrically wrinkled perio- 

 stracum, generally worn off in spots exposing a chalky white surface under- 

 neath. The anterior side is shorter, with a rounded margin, the posterior 

 side longer, with an obscure posterior angle, narrowed towards the margin 

 which appears as if cut off or crudely truncated. Below the periostracum, 

 the surface of the shell is white, and where extensively corroded, the outer, 

 chalky layer may be completely destroyed revealing the more durable 

 inner, porcellaneous layer. The shell with the periostracum removed is 

 white with a shading of purple over the umbones and along the posterior- 

 umbonal slope, the interior white, sometimes faintly pink or peach-colored 

 in the umbonal cavity. 



Length 61.6 mm., height 54.1 mm., diameter 48.2 mm. Tumbez, Peru. 



This is a large, convex, rounded shell with a coarse, heavy, dark- 

 colored periostracum, usually deeply corroded over the umbones, the ex- 

 posed surface white and chalky. C. B. Adams noted that he found this 

 species living at Panama in deep, impalpable mud, under bushes, at high 

 water marl, and where a small stream emptied into the sea. 



Range — Mexico to northern Peru. Costa Rica: Puntarenas (Hertlein 

 and Strong). Panama: Panama City; Bahia Honda, El Lagartillo. Colombia: 

 Isla del Gallo. Ecuador: Cojimenes; Santa Elena. Peru: Tumbez. 



Genus NEOCYBENA Fischer and Crosse. 1893 

 Type species by subsequent designation, H. B. Baker, 1930, Cyrena 

 nicaraguana Prime, 1869 {=N. insignis Deshayes, 1854). Central and South 

 America. 



Shell subtrigonal, with slightly anterior, prosogyrate beaks, porcel- 

 laneous and generally quite sohd texture. Hinge wide with strong teeth, 

 especially the lateral set and their sockets, the anterior lateral tooth placed 

 a little closer to the cardinals than the posterior one. The surface of the 

 valve is concentrically marked, either by fine or by coarse sulci, the whole 

 covered by a shiny, olivaceous to nearly black periostracum, usually worn 

 off the beaks. Color of shell below the periostracal cover is white, lightly 

 or heavily flushed with purple often in rayed bands; also the interior. 

 Pallial line with a small but distinct sinus just in front of the posterior 

 adductor scar. Adductor impressions are distinct and of equal size. Ventral 

 margin smooth. 



Species of this genus are distinguished from those of Polymesoda by a 

 more trigonal and usually a more solid shell. The periostracum is smooth 

 and polished. The pallial sinus is short and well developed. 



A large number of species of Neocyrena have been described from tropi- 

 cal America, mostly by Deshayes, often from unknown stations; the validity 

 of many of these forms as distinct species cannot be properly assessed at 

 this time or until much larger collections showing range of variation and 

 distribution become available. 



Neocyrena fontainei (d'Orblgny) Plate 27. figure 4 



Cyrena fontainei d'Orbigny, 1844, Voy. Am6r. Mirid., vol. 5, 3d partie, Mollusques, p. 



