256 GILBERT C. BOURNE. 



as that in Enigma, but there is also this difference between 

 the two genera ; that whereas in Anomia the rotation has 

 affected the position of the animal with respect to its shell, 

 the latter retaining its symmetry, in Enigma the inequality 

 of growth has affected the shell as well as the contained 

 animal, and both are asymmetrical to the same degree, as 

 may be seen by a comparison of pi. 15, fig. 1, with the text- 

 figure. Notwithstanding the change of symmetry, it will be 

 convenient to describe the upper and neai-ly straight margin 

 of the shell as dorsal, the lower convex margin as ventral, 

 and the two ends as anterior and posterior respectively. 



The impressions of the posterior adductor muscle, the 

 anterior and posterior extractor muscles of the foot (here 

 serving as retractors of the byssus) and of the large byssus 

 muscle are shown in the text-figure and do not require special 

 description. But it should be noticed that there is a fifth 

 muscular impression, pl.m., which has been overlooked in 

 previous descriptions of this genus, but Woodward (17) 

 describes and figures a similar muscle under the name of the 

 branchio-pallial muscle in Placuna placenta. This impres- 

 sion marks the point of attachment of a specialised band of 

 pallial muscles running forward and downward in the left 

 mantle lobe. 



General Ana.tomy, 



Fig. 1, pi. 15, is a representation of the animal lying in 

 the left valve of the shell. The right mantle lobe has been 

 largely cut away, and the left mantle lobe is obviously con- 

 tracted by the action of preservative fluids; otherwise the 

 animal is shown in its natural position. As has been explained 

 above, ^Enigma has not undergone so great a degree of 

 rotation as Anomia, and retains its original symmetry with 

 regard to the shell. We find accordingly that pallial cavity 

 or sinus lying dorsad of the (posterior) adductor muscle, and 

 containing the posterior recurved free ends of the gills, is 



