MUSEUM OF COMPARATIVE ZOOLOGY. 303 
larger. In M. angularis the posterior keel runs from the umbones to the pos- 
terior ventral angle of the rostrum ; in paucistriata the rostrum is posterior to 
both the keels. 
The shell of the present species is so fragile as to give way under the slight- 
est pressure. The soft parts hardened by alcohol were stronger than the shell, 
and offered some observations of interest. They were apparently in a perfect 
state of preservation. 
The outer edge of the mantle was plain, with a covering of epidermis as in 
Mya. Around the siphonal opening, which externally is single, were numer- 
ous tentacular filaments and several ocelli. The opening for the foot is very 
small, a mere short slit without ornamentation. On looking from above at 
the animal deprived of its shell, we see a globular body corresponding to the 
cavity of the valves, divided by a membranous and fleshy horizontal partition 
into upper and lower halves or subequal portions. The lower half constitutes 
the peripedal chamber into which the pedal and siphonal orifices open. The 
upper half contains the viscera, which, however, do not fill it, and the muscles. 
From above we see the floor or septum between the two chambers surrounded 
by a strong muscular band attached by its edge to the thin mantle and by 
upward radiating fibres like a drumhead inside of a drum; this muscular band 
resembles a sphincter, and is produced to the ends of the shell, where it is 
attached inside of each adductor ; the course of its roots being vertical, while 
the adductors lie in a horizontal plane immediately outside of the former, so 
that, when visible, the adductor scars and the others adjoin. In the middle 
line of the back are visible the cesophagus and alimentary canal, passing as 
usual through the heart, and through a small dark greenish liver-mass on whose 
dorsal surface are two small bunches of oval tubules, perhaps genitalia, and a 
whitish superficial subdendritic layer, probably the organ of Bojanus. From 
the centre of the visceral mass a mesenteric band descends to the centre of the 
floor or septum. In advance of this is the base of the foot, with a slender 
pedal muscle. 
Reversing the animal we see the septum has a sparsely tuberculous surface 
(smooth in C. glacialis Sars). Anteriorly is the mouth, simple, without palpi 
or gills, opening between two vertical mesenteric bands of tissue. Immedi- 
ately behind the oral orifice is the foot, small, subcylindrical, set in an excava- 
tion in the septum on a very short constricted peduncle and without any 
byssal groove or byssus. Posteriorly is the cylindrical opening of the siphons 
which are not separated from one another except by a delicate protrusile sep- 
tum, pierced for the two openings and situated within the single orifice of the 
mantle. No gills are visible anywhere unless the fleshy tuberculous ventral 
surface of the horizontal septum fulfils that office. A similar state of things 
in the main was observed in Cuspidaria glacialis Sars, and C. obesa Loven, in 
which, however, the foot was thorn-shaped, not cylindrical, and the visceral 
mass filled or nearly filled the upper chamber, 
