BLOOMER : ON THE BRITISH SPECIES OF THE GENUS SOLEN. 37 



its widest part. It curves a little dorsally, is bilaterally symmetrical, 

 and is enclosed ventrally by the concrescence of the edges of the 

 mantle lobes, with the exception of the apertures at the anterior and 

 posterior ends, and a fourth aperture situated nearly at the centre of 

 the ventral surface. 



The periostracum passes from the outside of the shell to the edges 

 of the mantle lobes, to which it adheres. 



The pallial muscles form a deep band along the margin of the 

 mantle lobes, and at the anterior end, surround the pedal aperture, 

 through which the foot is protruded. At the posterior end the muscles 

 assume a more circular condition, and give rise to the siphon con- 

 taining the afferent and efferent chambers. 



On separating and turning back the left mantle lobe, it is seen 

 that the foot projects from nearly the centre of the ventral surface of 

 the animal and proceeds in an anterior direction. A little anterior to 

 the foot is the mouth, and in the front of the mouth the broad anterior 

 adductor muscle (PI. ii, fig. ?, A. A.). On each side of the viscero-pedal 

 mass are the labial palps, and commencing between and passing 

 posterior to them, are the two pairs of gills which extend to the 

 siphon. The anus opens into the cloacal chamber from a free portion 

 of the rectum, behind the posterior adductor muscle. 



The siphon consists of two separate chambers, the upper one the 

 exhalent, the lower one the inhalent. The free portions of the 

 siphonal chambers are short, and separate from each other, both are 

 encircled with a fringe of pale tentacles, and at the distal end of each 

 chamber are two flaps forming the valve. 



The fourth aperture is an elliptical opening which narrows very 

 much at the inside edge of the mantle lobes ; around the inside of it, 

 but near the outer edge, is a row of tentacles, those on the one side 

 alternating with those on the opposite side. The tentacles and surface 

 of the lobes bordering the opening are of a lighter or paler colour. 

 On the inside of each mantle lobe is a groove passing dorsally from 

 this aperture towards the foot, and in it lie the distal portions of the 

 labial palps. 



Solen siliqua, L. (PI. ii, fig. 3). 



This species very closely resembles S. ensis in its external characters, 

 only it is Farger, and quite straight along its dorsal surface. 



Solen marginatus, Pult. and Don. (PI. ii, fig. 4). 



Externally this species exhibits many points of difference when 

 compared with *S'. ensis. It is shorter, and, like S. siliqua, straight 

 along its dorsal surface. The pallial muscles are more strongly devel- 

 oped, while at its anterior end in the muscular part of the mantle 



