436 Annals of the Carnegie Museum. 



to nearly elliptic without any angles, except a slight, rounded one at 

 the scutum, supero-anterior slope not. or little, marked, anterior end 

 rather broadly rounded, posterior end rounded to subtruncate ; beaks 

 slightly behind the middle, broad, rounded, moderately projecting over 

 the hinge-margin ; surface shining to somewhat dull and with a slight 

 silky gloss, under the microscope distinctly rugulose, with fine, irregu- 

 lar, somewhat sharp and crowded strire and a few slightly marked 

 rest-lines; color pale corneous, shell thin, translucent; hinge about 

 three-fourths the length of the mussel, slight, with a somewhat broad 

 plate; cardinals rather small and slight, well up on the plate, the right 

 curved with its posterior part bifid, a somewhat narrow, but deep, 

 excavation below it; left anterior somewhat oblique, curved to angu- 

 lar, its edge pointed, posterior oblique, slightly curved, "covering" 

 only a small part of the anterior; laminae slight, projecting inward 

 but little, the right inner both with somewhat proximal, small, pointed 

 cusps, the outer both short and small ; left both with the cusps about 

 median, small, pointed, somewhat abrupt ; ligament rather short and 

 thick. Long. 4.3; alt. 3.6; diam. 2.6 mm. 



Habitat: Orcas Id.. San Juan County. Washington. Collected by 

 Carl C. Engberg in 1919. Type C. M., No. 9242 and seq. ; paratypes 

 in the collection of Mr. Engberg. 



There were about fifty specimens in the lot. ranging from quite 

 young to apparently mature ; one mussel opened contained six nepionic 

 young 1.5 mm. long. Some specimens are slightly more elongate, with 

 the surface more polished, whitish to straw-color, but probably not 

 distinct. 



At first glance this PisidiiDii does not show very strongly marked 

 features, but upon closer examination it proves peculiar as to its shape 

 and hinge. It may be noted that the right cardinal is placed close to 

 the edge of the valve (nymph), and this is in accord with the short- 

 ness of the left posterior, only its posterior (inferior) part being 

 developed. 



As to shape, P. orcascnsc is similar to P. p-usillitiii Jenyns from 

 Europe ; whether they may be really related to each other must be 

 ascertained later. 



10. Pisidimn prognathum n. sp. Mussel of rather small size, 

 oblique, little elongate, rather well and evenly inflated, anterior part 

 slightly longer and directed downward ; superior part rather long. 



