MUSEUM OF COMPARATIVE ZOOLOGY. 445 



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 simple 

 oblique upper ends of the septal orifices. What can their office be? I suppose 

 that they serve to admit fresh water to the upper chamber, which I believe to 

 be utilized in some, if not all, instances as a marsupium. It is probable that, 

 by suitable muscular contractions, the septum will operate somewhat like the 

 washer of a pump- valve, and that the upper chamber can be filled or emptied 

 of its contained water at will. 



I believe the septum in Cuspidaria to be homologous with the ordinary 

 siphonal septum, only more prolonged; and that its muscular tissue is the 

 equivalent of the siphonal retractors of ordinary Pelecypods. I do not regard 

 it as in any manner homologous with the normal ctenidia. 



Mijonera paucistriata (I. pp. 302, 303). To the description of the soft parts 

 of this extremely fragile and delicate form already published, several points can 

 be added from the examination of a fresh specimen recently received. The 

 only correction to the original description relates to the opening of the anal 

 siphon, which is a minute circular orifice in a delicate membranous area which 

 in life probably projects in a dome-like manner, but in alcohol appears tense 

 and flat. The opening is into the upper portion of the peripedal chamber, of 

 course, as in the other species. That which I took for the anal opening in the 

 first specimen examined was an accidental lesion, while the true anal opening 

 from its minuteness was overlooked. 



The mouth, as stated in 1886, is a simple opening without palpi. The latter 

 are represented, if at all, by a delicate slightly elevated ring of tissue which 

 surrounds the circular mouth. The absence of gill laminae is fully confirmed. 

 The septal orifices on the ventral surface are hardly observable without the 

 closest scrutiny, though easily visible on the dorsal surface of the septum. 

 There are eight, as in the Cuspidaria patagonica, and their lips, slightly elevated, 

 usually appear triple, so as to give a triangular aspect to their junction. When 

 sounded by a delicate probe they appear subtubular. 



The muscular tissue of the septum is concentrated in two bunches of coarse 

 fibre-bundles, which radiate from the posterior outer corners of the septum, 

 suggesting that the fibres, usually devoted to retracting in a nearly vertical 

 plane the siphons toward their angular insertion (pallial sinus) on the shell, 

 are here spread in a horizontal plane. Beside the fasciole of fibres at the 

 corners, there is a loosely arranged central bundle behind the foot, while the 

 rest of the septum is more thin and fibrous, and the vertical roots of the septal 

 muscles far less strong and prominent in proportion, than in Cuspidaria. The 

 arrangement of the fibres of the muscular tissue is singularly loose, and in the 

 central area irregular, — quite different from the solid tissue of the septum in 

 Verticordia, or the compact bands observable in Cuspidaria. 



The most noticeable feature in this specimen was the condition of the 

 ovaries. These ramified over the posterior part of the visceral mass, terminat- 

 ing in bifurcated or trifurcated sacs, largest at their distal extremity, and some- 

 what fig-shaped. These were crammed with ova and projected from the 



