54 



perceptible ; this appearance is by no means unfrequent, as scarcely 

 any slide I have mounted is deficient in furnishing one or more such 

 facts. 



I must not omit to mention, that I discovered the fact of the cen- 

 tral aperture being covered internally by a sort of membrane, in con- 

 sequence of Mr. Legg having drawn my attention to an appearance 

 presented by one shell mounted in balsam which he has, in which 

 the membrane is present, but slipped a little from its place. 



The various appearances described in the foregoing paper are ac- 

 curately represented in the drawings, which, by the kindness of 

 Mr. Legg, as I previously mentioned, I am enabled to lay before 

 you. 



The objects themselves I shall have the pleasure of presently ex- 

 hibiting, and the accompanying slides I purpose depositing in the 

 Society's cabinet. 



DESCRIPTION OF PLATE XI. 



Fig. 1. The external membrane, as seen when detached from the inner frame- 

 work, or when viewed from the outside of the shell as an opaque 

 object. 



Fig. 2. The inner frame-work as exhibited on a black disk as an opaque object. 



Fig. 3. One of the two annular valves. 



Fig. 4. The membrane and "frame-work united, as seen by transmitted light; 

 200 linear. 



Fig. 5. The same, more amplified ; 500 linear. 



