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 paucisfriata 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 oflfered 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 tentacidar iilaments 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 oesophagus 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. 



