PANAMIC-PACIFIC PELECYPODA 4S9 



Length 19.2 mm., height 15 mm., diameter of both valves 12.5 mm. 

 Off Clarion Island (Hertlein and Strong). 



When fully grown, this fine species is easily recognized by its unusual 

 shape and by its large, conspicuous, nepionic shell capping the umbones 

 of toth valves. Fresh specimens are generally brightly colored, rayed with 

 red but plainly marked shells may also occur. Hertlein and Strong recorded 

 this species from several localities in the Gulf of California and southward 

 to Costa Rica. 



Range — Gulf of California south to Costa Rica. Gulf of California. 

 Costa Rica: Gulf of Nicoya. 



Family GASTROCHAENIDAE 

 Borers in limestone, coral, and the thick wall of shells, forming flask- 

 shaped excavations or if found in softer material such as sand, they develop 

 a calcareous tube which covers the small valves and siphons. The shell, 

 itself, is small to medium-sized, thin, equivalved and inequilateral, elongate- 

 mytiliform, the ventral-posterior side so widely open that the valves are 

 in contact only along the dorsal side and at the ends. The hinge is edentulous, 

 the ligament external, attached to a linear groove above a weak, nymphal 

 ridge. In some forms, there is a shelflike lamina under the beaks extending 

 into the umbonal cavity; its function is believed to be that of a myophore. 



The Gastrochaenidae are mainly borers in limestone rock, coral, shell, 

 lining their excavation with a tube which at the exit may project above 

 the surface in the shape of a narrow neck or it may lay prone on the surface 

 in the shape of an irregular, gourdlike tube. Gould considered this gourd- 

 Hke growth as characteristic of some species and proposed the name Cucur- 

 bitula (type species C. cymbium Spengler = laginula Lamarck), but later 

 workers have shown that this sort of tube is accidental and is formed only in 

 such Gastrochaena which have become affixed to a thin-walled shell into 

 which they could not bore. 



With two genera characterized as follows: 



I. Surface of valves with a plain sculpture of concentric growth lines only. 



Genus Gastrochaena 



A. Cardinal margin thickened and carrying an internal lamina or myo- 

 phore extending towards the umbonal cavity. 



Subgenus Gastrochaena 



B. No myophore, the dorsal margin plain and unthickened. 



Subgenus Rocellaria 



II. Surface marked by a deep ray or sulcus running from the beak to the 

 posterior-ventral margin, the posterior surface above sculptured wi*h 

 deep, transverse ridges, the posterior margin more or less sharply 

 tuncated. 



Genus Spengleria 



Genus GASTROCHAEXA Spengler, 1783 



Type species by subsequent designation, Bucquoy, Dautzenberg, and 

 DoUfus, 1896, Gastrochaena cuneiformis Spengler. Recent, West Indies. 



Shell small or of medium size, with two, thin, elongate, wedge-shaped 

 valves, the ventral gap large. Beaks nearly terminal, the anterior side short 



