280 REPORT ON ALBATROSS MOLLUSCA DALL. 



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 banging vertically is short, wide, and 

 represents the languette iu Oardium; the lateral processes are some- 

 what longer and obliquely set, the whole forming a large subtriaugular 

 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 behind the edge of the foot, 

 where the surface in all the forms with a muscular septum is elevated 

 like a wave rising about a solitary rock. The foot is slender, elon- 

 gated, slightly geniculate, with a small byssai groove behind. Imme- 

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

 spicuous 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 auterior commissure of the lobes of the man- 

 tle. Here a narrow homy or chitinous gusset strengthens the com- 

 missure, 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 iu the thickest 

 part of the muscular portion 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 communicates with the 

 upper chamber. 



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

 careful sectioniziug I found the third pair imperforate in a fairly grown 

 specimen of G. rostrata, while in several young specimens the two pos- 

 terior pairs seemed imperforate. In a specimen of G. arctica var. glacialis 

 I found five orifices on each side, showing that the number is not inva- 

 riable. 



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

 in Getoconcha or Poromya. The office of a gill must, therefore, as sug- 

 gested in 1886, by me in the first part of my Blake report (p. 303), be 

 performed by the surface of the septum or by the lobes of the mantle. 

 This is a very different view from Dr. Pelseneer's idea that the septum 

 is itself homologous with the ordinary gills of Pelecypods. 



By cutting the lobes of the mantle away, and carefully turning back 

 the septum as a whole, extracting the foot from its socket, we see the 



