136 POPULAR BRITISH CONCHOLOGY. 



see-saw movemeut to its shelly and vibrates its tentacles, 

 which, like the snail, it uses as feelers. 



III. Adeorbis subcarinata takes its name from a prett}^^ 

 minute, white, spreading shell, with several angular 

 ridges or keels. 



IV. ScissuRELLA ciispata is named from a very minute, 

 microscopic, white shell, which has a slit in the outer 

 lip of the aperture. Continuous with this slit, and run- 

 ning round all the whorls, is a groove, partitioned off 

 by cross-bars. 



A fossil genus of large shells, named Pleiirotomaria, pre- 

 sents a similar structure, which I am much inclined to think 

 analogous to successively-closed holes of the Haliotis ; the 

 slit in the Scissnrella answering to the last open hole of the 

 Haliotis, and the pits in the spiral groove of the former 

 agreeing with the block ed-up apertures of the latter. This 

 cannot, however, be known with certainty until the animal 

 of our Scissurella can be examined, and then we may per- 

 haps have a new light thrown on the habits of the extinct 

 group of mollusca to which we have referred. 



