Occasional Papers of the Museum of Zoology 27 



Plagiola {Artonaias) opacata (C. and F.). subspecies new 

 ?. — One specimen from the Rio San Juan quite closely 

 approximates P. opacata, but has only one right pseudocardinal 

 (the posterior). With it probably belong two or three much 

 larger valves, all badly eroded and broken, but which show 

 a considerably heavier and more arched hinge plate. These 

 shells perhaps represent large subspecies (river form), but the 

 material is too scanty and poor to justify a description. In 

 many ways this form somewhat resembles P. sallei (C. and F.) 

 (which would have the priority over opacata), but the latter 

 appears to have higher and fuller beaks (fig. 32, plate VII, is 

 comparable in this regard), and a trigonal, right posterior 

 pseudocardinal. Simpson (1914) also speaks of the absence 

 of radial striations, but they are difficult to find, or are com- 

 pletely lacking, in the epidermis of some specimens of P. opa- 

 cata. These large specimens also resemble, to a certain degree, 

 some of the older shells of A. sapotalensis (Lea), but the latter 

 may be quite easily separated by their vertical right pseudo- 

 cardinals, which are trigonal in shape, and by the zig-zag rays. 



Lampsilis rovirosai Pilsbry sanjuanensis, new subspecies 

 Plate VII, figs. 39-42 



Fourteen specimens, including odd valves, from the Rio San 

 Juan (H, vii, c). 



Shell rather large, subrhomboid to obovate; rather elongate, 

 subinflated; posterior ridge well rounded and with tw^o radiate 

 sulcations on postero-dorsal slope ; hinge-line rather short and 

 quite straight; beaks rather full and well developed (especially 

 as compared to A. explicata) ; beak sculpture (from remains 

 in two younger specimens) apparently consisting of five or 

 six, rather coarse wrinkles, scarcely looped ; epidermis radially 

 striate, olive-green in younger specimens, shading to yellowish 

 at beaks and at the edge of the shell, blackish in old specimens; 

 growth-lines fine and regular, giving the shell a soft, dull finish ; 

 left valve with two almost horizontal pseudocardinals, lamel- 

 late to heavy and jagged, but always compressed, and with two 



