34 NILS HJ. ODHNER, MOLLUSCA. 
the right one in Pseudochama. The pericardial tubes emanate through long and 
narrow oscular ciliated funnels from the lateral corners of the pericard. The sequent 
portion of them is widened and furnished with folded walls; then the tubes narrow 
again and extend, either surrounded by the outer sacs (Chama) or uncovered by them 
on their median side (Pseudochama), upward till they debouch into the outer sacs, 
earlier in Chama than in Pseudochama. The lobation of the external sacs is minute 
and the lining cells are larger (bladder-like) in Chama than in Pseudochama. The 
nephridia open out somewhat below the genital organs, distinctly separated and 
opposite to the genital pores. 
Muscles. As a consequence of the reduction of the foot, all Chamidae have 
the foot retractors vestigial, but they still retain different degrees of development in 
the different species, thus in Ch. lazarus the retractors are much more conspicuous 
than in Ch. reflexa and Pseudochama cristella and reach the shell, where they are 
inserted, which they are not in the last-named forms. 
FAM. SAXICAVIDAE. 
Saxicava arctica Linn&é (var. australis Lamarck). 45 miles W.S. W., 70 feet 
(Js), 1 spt, 1.14 mm. Tare and May (1901) consider Lamarck’s 8S. australis, to 
which the present specimen belongs, as identical with S. arctica and S. pholadis Linn. 
FAM. CHAMOSTREIDAE. 
Chamostrea plicifera n. sp. (PI. 1, figs. 26—32, text figs. 5, 6). Shell sessile, 
its right valve being attached by means of its anterior half, and the posterior half of 
it being raised from the underground; its shape triangularly ovate, and its colour 
light yellowish green, with three dark olive brown lines running from the umbones 
to the posterior margin of the upper as well as of the lower valve. Upper valve 
operculiform, its superior margin highly arcuated, the posterior margin a little 
longer and less curved, separated at an angle from the short inferior side; anterior 
margin stretching perpendicularly up to the umbo and separated from it by a small 
sinus continuing as a broad and short furrow beneath the umbo. The upper valve 
sculptured outwardly with two furrows close to the upper margin and a diagonal 
curved impression dividing the valve into a convex anterior and a flattened posterior 
part; the anterior half sculptured by very fine raised irregular striae or rows of gran- 
ulae and concentrically irregularly rugated, with some distant lines of growth. The 
lower valve sculptured with two furrows close to the upper margin, some lines of 
erowth and extremely fine granulae, for the rest smooth. Inside of the shell nacreous; 
the upper valve in its upper part radiately plicated, the lower valve smooth. Pallial 
line simple; anterior muscular scar much lengthened, the posterior one short. Liga- 
ment represented both by a feeble thickening of the exterior cuticula and by an 
interior resilium transformed into a free ossicle attached into a small hole on the 
inside of each valve. Hinge consisting of a small knob-like tooth in the upper valve, 
fitting into a socket of the lower one, bounded by a raised margin. Dimensions: 
