OF CONCHOLOGY. 



317 



off to a very thin edge, which, in its turn, is overlapped by the 

 thick columeHar margin of the preceding plate. The usual form 

 of the operculum of such Strepomatids as I have examined is 

 oval or subtriangular, embracing a spiral of three or four turns, 

 the polar point being near the base, but they are subject to such 

 great deviation that, of large numbers belonging to t^e genera 

 G-oniohasis, Euryccelon, Schizostoma and Leptoxis, the usual form 

 is that of a sub-spiral, the central lamina exhibiting the spiral 

 character, having become abraded or worn off. In the genera 

 Tnjpanostoma, Strephobasis, Lithasia and io, the operculum is 

 similar in structure to the genera before mentioned, the shape in 

 Trypanostoma being usually oval, while in the latter it is sub- 

 triangular. In these genera it is stouter in proportion to the 

 size, more coarsely laminated, less elastic and with an oval de- 

 pression at the polar point, its small size enabling the animal to 

 retire to a position far within the aperture, where the closure 

 becomes perfect. It would seem that the central portion of the 

 operculum, having been secreted in the earlier stages of the ani- 

 mal's existence, the plates of Avhich it is composed are thinner 

 and more elastic at this point, and owing to the continued action 

 of the foot in closing the aperture, the concavity is produced 

 which is observed in these genera. In several genera the oper- 

 cula, when dried, present a more or less twisted appearance, 

 which is due to their varying thickness and the shape of the 

 aperture into which they are forcibly drawn by the animal during 

 their growth. 



In G-oniobasis Showalteri. Lea, and Leptoxis rubiginosa^ Lea, 

 this organ, in some individuals inhabiting still water, presents a 

 continuous growth. The earlier formed laminae, of which it is 

 composed, is pushed off from the proligerous lobe in the direction 

 of the labium, forming a free margin as in the human nail ; in such 

 specimens the spiral character of the operculum is absent, th s 

 organ forming a long wedge-shaped ribbon, tbe free end of which 

 coils inward, partly by the force of its own elasticity, but chiefly 

 from the direction given by the imbricated arrangement of its 

 laminae.* In many specimens of operculae the polar point men- 

 tioned by authors is not the centre of the spiral, the centre hav- 

 ing been broken off in the continuous growth just alluded to, but 

 that which is taken for the polar point is a folding or partial 

 twisting upon itself near the base of the aperture of each plate 

 of which it is composed, constituting, in the great majority of 

 instances, the form met with in the genera Gromobasis, Euryccelon, 

 Schizostoma and Leptoxis. With these genera the margin of the 



* See Lea's Obs., vol. 9, page 42, pi. 24, fig. 4. 



