444 BULLETIN OF THE 



The siphonal septum in this species divides the cavity of the shell unequally, 

 the upper portion being smaller than the lower. In the upper, sustained 

 especially by a median fibrous mesenteric band, is suspended the visceral sac. 

 Viewed from above, it is subcordate in profile; from the side it seems acutely 

 ovoid. It occupies, as contracted by alcohol, about half the cavity above the 

 septum. The valve of the anal siphon is represented above the septum by a 

 thin vertical wall of membrane pierced by a relatively small simple central 

 orifice. The valve of the branchial siphon below the septum is composed of 

 three rather thickish processes; one, hanging vertically, is short, wide, and rep- 

 resents the languette in Cardium; the lateral processes are somewhat longer 

 and obliquely set, the whole forming a large subtriangular opening with three 

 partially overlapping curtains. Passing backward on the ventral surface of 

 the septum, aside from the streakiness due to the fibrous coarse muscular tissue, 

 there is a distinct narrow median depression behind the foot, except just be- 

 hind the edge of the foot, where the surface in all the forms with a muscular 

 septum is elevated like a w^ave rising about a solitary rock. The foot is 

 slender, elongated, slightly geniculate, with a small byssal groove behind. 

 Immediately in front of it the surface is depressed about the small and in- 

 conspicuous mouth. Here the anterior palps are almost wanting, but the 

 posterior, though abnormally small, are elevated above the surface and strongly 

 transversely striate. In front of the palps is a strong ridge of tissue, behind 

 the anterior commissure of the lobes of the mantle. Here a narrow horny 

 or chitinous gusset strengthens the commissure, above which is a sort of 

 pocket or shallow indentation, above which the external margin of the mantle 

 finally joins. The gusset is narrow, concave in the middle, with its ends 

 spatuliform, and shows brown through the white tissues, like the jaw of a 

 Gastropod. 



If the surface of the septum near the foot be closely scrutinized, there will 

 be seen on each side four slight prominences. The anterior pair are on each 

 side of the mouth, the second and third pairs by the sides of the foot, the 

 fourth behind the foot, all situated in the thickest part of the muscular por- 

 tion of the septum. The posterior pair have two lips, the others three, to each 

 prominence, and on gentle pressure with a fine probe it will be found that a 

 small circular orifice passes somewhat obliquely through the septum and com- 

 municates with the upper chamber. 



These passages are not always complete, however, for by means of careful 

 sectionizing I found the third pair imperforate in a fairly grown specimen of 

 C. rostrata, while in several young specimens the two posterior pairs seemed 

 imperforate. In a specimen of C. arctica var. glacialis I found five orifices on 

 each side, showing that the number is not invariable. 



The lips to these orifices are not prominent, much less so, indeed, than in 

 Cetoconcha or Poromya. The office of a gill must, therefore, as suggested in 

 the first part of this Report (p. 303), be performed by the surface of the sep- 

 tum, or by the lobes of the mantle. This is a very diff'erent view from Dr. 

 Pelseneer's idea that the septum is itself homologous with the ordinary gills 

 of Pelecypods. 



