SHELLS. 235 



The genera are as follows : — 1. Genus Vermetus, 

 ■which is represented by the Vermetus lumhricalis. 

 2. Genus Magilus. The Magilus antiquus is the 

 only species known in Bermuda. The animal esta- 

 blishes itself in the excavations of madrepores ; and 

 as the coral increases round it, the Magilus is obliged, 

 in order to have its aperture on a level with the 

 surrounding surface, or near it, to construct a tube, 

 the growth of the coral determining its length. 



VIII. Gasteropoda Scutihranchiata. — The shells in 

 this order are very open, having no operculum, and 

 the greater number are not in any degree spiral : 

 so that they cover their animals, and particularly 

 the branchiae, in the manner of a shield. 



The following are the genera belonging to this 

 family: — 1. Genus Fissurella have the perforation on 

 the top of the shell, very much resembling a key- 

 hole. This aperture is for the purpose of respiration, 

 as the water thus commimicates to the branchial 

 cavity, which is placed, something like that of the 

 Doris, on the forepart of the back. The Fissurella 

 Grceca is a familiar example of the genus. 



2. Genus Emarginula, the Emarginula fissura 

 being an example. 



IX. Gasteropoda Cyclobranchiata. — The genera of 

 this order are the following : — 



