28 INTRODUCTION. 



Plate III. fig 7 and 8, O O. This is termed the Inner 

 Lip by some authors. \ 



Outer Lip — The expansion, or continuation of the body : 

 of the shell, on the left margin of the aperture, also lined I 

 with the glossy process of the aperture. Plate II. fig, 7, -N'; j 

 and Plate III. fig. 8, NNN. The latter is an example of j 

 the alated or winged shells. | 



Operculum, or lid. This is oidy an appendage to the 

 turbinated or spiral shells affixed to the foot of the animal, i 

 sometimes of a testaceous, in other cases of a horny or car- 1 

 tilaginous substance. It acts as a door or lid, and is calcu- | 

 lated for the protection of the animal, when it retires within | 

 its dwelling from the intrusion of its enemies, it being j 

 adapted to the shape of the aperture, which it closes nicely i 

 up. As exemplified in the horny operculum of the common ! 

 Periwinkle, (the Turbo littoreus of Linnaeus,) it is of a ; 

 brown horny appearance; it is also to be found in the large I 

 Whelk, which is common in our seas, and sold in fish- 

 markets as food. Plate III. fig. 4. 



Spire consists of all the whorls of the shell, except the | 

 lower one, which, as before observed, is termed the body of 

 the shell. Plate II. fig. 12, D; and Plate III. fig. 7 and 8, 

 DD. 



This spire is a prominent feature of the Univalve; and 

 upon its being lengthened or elevated, shortened or de- 

 pressed, &c. depends much of the generic and specific 

 definition. Adanson, in his ' Natural History of Senegal,' 

 printed at Paris in 1757, says that the external character of 

 the spire varies according to the plane it turns upon, which, 

 he observes, may be horizontal, cylindrical, conic, or ovoid. 

 At the same time, he admits that there are a great many 

 intermediate forms, which cannot properly be defined. 



It is a remarkable circumstance, that many of the young 



shells have not the same number of turns or wreaths as the 



adults; from which it would appear, that the part of the 



I animal nearest the apex, never increases in size. The num- 



i ber of wreaths cannot at all times be depended upon. A 



full-grown shell may, however, be known from the outer- 



