CLASS PELECYPODA 
35 
Type in U. S. N. M., No. 212499. Type locality, Station 3346, off 
Tillamook Bay, Oregon, in 786 fathoms. 
Range. Known only from the type locality. 
Yoldia siliqua Reeve, 1855. 
Last of the Arctic Voy. t 2:396; pi. 33, fig. 8. 
Shell oblong-subquadrate, thin, ventricose, covered with an olive or 
blackish brown epidermis; posterior side widely angular, acuminated 
above, ventral margin obliquely truncated at the end, dorsal margin 
slightly concave, depressed; umbones rounded, elevated, ventral margin 
rather straight in the middle; anterior side short, rather rounded. (Reeve.) 
Type in British Museum. Type locality, Arctic Ocean. 
Range. Arctic Ocean, Point Barrow. Also Atlantic. 
Yoldia intermedia Sars, 1865. 
Plate 1, figs. 7, 10. 
Fos. Dyre. Quatern. period, p. 38; figs. 92, 96; or tablet 4, figs. 9, 9a. 
Testa tumida, forma sub elliptica, duplo fere longior quam altior, 
extremitate antica breviore et rotundata, postice producta, leviter at- 
tenuata, apice obtuso, margine ventrali medio parum arcuato postice 
oblique ascendente, dorsali postice fere horizontali, umbonibus marginis 
et prominentibus non nihil ante medium sitis. Valvulse tenues, lavissimse, 
epidermide luteo-virescente nitidissima tectse, denticulis cardinalibus 
utrinque 12—16. Long, usque ad 12 mm. (Sars.) 
Type in Museum Christiania. Type locality, Vadso, Norway. 
Range. Bering Sea to Norton Sound, Circumboreal. 
Genus PLEURODON S. Wood, 1840. 
This very remarkable little genus combines characters which recall 
Nucula, Limopsis, Tindaria, and other genera, with features peculiar to 
itself. The typical species is characterized by a shell externally resembling 
Nucula, but having a structure much less nacreous. The anterior side of 
the hinge line is short, the cardinal border externally is produced and 
angulated, and in an excavation under this little angle lies the ligament 
which in a normal and perfectly preserved specimen is nearly or quite 
covered by the margin of the valves. In most specimens this covering, 
being extremely thin, is eroded or broken away, so that two valves in 
opposition show a small oval pocket in which the ligament was originally 
contained. . . . The cardinal plate in all the species is rather broad and 
terminates in the left valve with a prominent lateral tooth which is received 
into a corresponding depression in the plate of the opposite valve, the edge 
