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



lobes which encircle the pedal aperture, is a groove, in which the 

 constricted part of the shell lies, from which character arises the name 

 of the species. The most important feature, however, appears to be 

 the non-existence of the fourth aperture, and of the labial grooves on 

 the inside of the mantle lobes. The diminution in the length of the 

 animal, when compared with S. ensis and S. siliqua, is seen to be in 

 the portion anterior to the foot. Just in front of the mouth, the mus- 

 cular portion of the ventral integument terminates abruptly, and with 

 the lips formed by the fusion of the labial palps, projects anteriorly. 

 The anterior adductor is not so broad as in the two preceding species. 

 It is also observable through the transparent ventral integument, that 

 the liver does not project anterior to the mouth. The foot is larger, 

 and has a more massive appearance. 



The siphon is much longer than in *S. ends, and in one piece, 

 though it shows clearly that at one time it was two distinct siphons. 

 Both internally and externally, the chambers exhibit a transverse and 

 longitudinal ribbing, and the free end of each is encircled with a row 

 of tentacles. 



The periostracum passes from the outside of the shell to the 

 mantle lobe, to which it firmly adheres, and then forms a free border, 

 particularly so at the anterior and posterior ends. 



The bases of the two inner gills are joined and enclose the cloacal 

 chamber for the greater portion of its length, whereas in S. ensis and 

 S. siliqua, I have found in the course of examination of a large number 

 of specimens, they are not so connected. 



Musculature. 



Solen ends, L. (PI. ii, fig. i and PI. iii, fig. 5). 



i. The Pallial Muscles. — The muscles along the edge of the mantle 

 lobe, or the muscles of the pallial edge (PL ii, fig. i, Pal. M'.), form a 

 deep and thick band. They are. composed of closely arranged bundles, 

 running in a ventral direction, at right angles to the mantle edge, and 

 become closer as they approach the line of concrescence of the two 

 lobes, where the thickness is further increased by another band of 

 muscles traversing each lobe parallel to the pallial edge. The mantle 

 lobe attains its maximum thickness just ventral to the line of its ad- 

 herence to the shell. At the anterior end, the pallial muscles form a 

 circular growth enclosing the pedal aperture (PI. ii, fig. i. Pal. M.), 

 and have two lateral portions, which are free and to a considerable 

 extent close the aperture on the withdrawal of the foot. The circular 

 muscular arrangement is the same at the posterior end, where, how- 



