392 SHELLS AND SHELL-FISH. 



an incipient modification of the angular dilation of 

 the hinge margin in Byssoarca, and several other 

 genera. 

 Auriform, or ear-shaped, as in Haliotis. 



Base. This term in regard to shells is variously applied : 

 generally speaking, it is used in opposition to the 

 apex or end of univalve shells, although, correctly 

 speaking, it is at this part where the head of the 

 animal protrudes. In such bivalves as adhere to 

 other substances by one of their valves, as in Spon- 

 dylus, Anomia, &c. that valve is termed the basal. 

 In unattached bivalves the term cannot be properly 

 employed. In reference to the whorls of spiral 

 shells, the last or largest is termed the basal whorl. 



Bivalve. Shells of the order Dithyra, having two valves. 



Bifid. Cleft, or double. 



Byssus. A bunch of silk-like fibres, by which many 

 bivalves adhere to other substances, without the shell 

 itself being deprived of motion. The Pinnae, or 

 wing-shells, are the most typical examples ; but 

 many other genera, as the Pearl and Hammer oysters 

 and even some of the muscles are thus provided. 

 When dried, the byssus of the large Neapolitan 

 Pinna is sometimes fabricated into gloves, but only 

 as objects of curiosity, since, as we know by per- 

 sonal inquiry, it is a most tedious and difficult 

 process. 



Callous. When there is a thickening of enamel upon 

 any particular part of a shell, resembling a tumour, it 

 is termed callous, or a callosity : this is observed 

 among spiral shells, in the inner lip of the Olives, 

 Naticse, and many others, and is very common near 

 the hinge of certain bivalves. 



Canal. A groove or gutter observable in different 

 parts of certain spiral shells, belonging to the Zoo- 

 phaga, or carnivorous tribe. In these the canal is 

 either lengthened, as in Murex, short as in Harpa, 



