PANAMIC-PACIFIC PELECYPODA 139 



usually in rock, lined with a tapered, auxilliary calcareous tube as in some 

 of the pholads. 



Lithophaga (Rupiphagra) hastasia, new species Plate 15, figures 5-5f 



The shell is narrowly elongate, bullet-shaped or modioliform, thin, 

 subcylindrical, evenly convex over most of the length, the anterior end 

 obliquely rounded with the small, low beak placed a short space behind, 

 the dorsal and ventral margins parallel for the most part, the posterior-dor- 

 sal half descending to form a tapered end. The greater part of the surface 

 is encrusted with a deposit of hme, relatively thin and filmlike on the an- 

 terior portion, coarse and heavy on the posterior side so as to form a thick- 

 ened rib along its middle bordered on each side by a furrow and prolonged 

 considerably beyond the end of one valve (usually the right) forming a 

 stout, solid spikelike plug. Periostracum is largely concealed by lime de- 

 posit but where exposed has a clear brown to dark-brown, nearly black 

 color. Interior is nacreous white. 



Length 24.5 mm., height 5.8 mm., diameter 6.6 mm. 



Holotype, Esmeraldas, Ecuador, ANSP 218931. 



Length 38.3 mm., height 9 mm., diameter 4.3 mm. (right valve). 



Paratype, Esmeraldas, Ecuador. 



In this species, the calcareous surface encrustation forms along the 

 posterior side a thickened rib which in one valve (usually the right) is 

 prolonged some distance beyond its end in the shape of a stout, spikelike 

 blade, often two-pronged at the tip; on the surface of the valve, this rib is 

 generally bordered on each side by a deep furrow or groove. The upper 

 part of the bore, usually in an argillaceous rock is internally lined with a 

 heavy, pipehke tube, wider below, much heavier and tapered at the top 

 with a small circular orifice. My observations on this structure are limited 

 to the bore and its internal tube. The fragile Lithophaga shell within is 

 seldom secured intact when the enclosing rock is broken into. 



Range — Panama to Ecuador. Panama: Guanico. Ecuador. Esmeraldas. 



Subgenus STU3IPIELLA Soot-Ryen, 1955 



Type species by original designation, Lithophagus calyculatus Carpenter. 



This subgenus is probably a synonym of Myoforceps. An unpublished 

 figure of Lithophagus calyculatus, prepared by Carpenter from the type, is 

 that of a shell closely similar to L. aristata if not identical with it. The shells 

 figured by Soot-Ryen as L. calyculata from the Mexican coast, probably 

 represent another species. 



Family DREISSENIDAE 



This family is represented in America by the genus Mytilopsis, a group 

 of fresh- or brackish-water species, both fossil and Recent, and often en- 

 countered in beach drift at localities situated near the mouths of fresh- 

 v/ater streams. They are properly speaking, not a part of the marine 

 Panamic-Pacific fauna. A species is common in some rivers of southwestern 

 Colombia and northwestern Ecuador, usually in the swifter portion of the 

 streams, attached in clusters to boulders and pebbles in the bed or living in 



