24< INTRODUCTORY. 



Outer Lip. The expansion, or continuation of the body of 

 the shell, on the left margin of the aperture, and is also lined 

 with the glossy process of the aperture. Plate II. fig. 7, N ; and 

 plate III. tig. 8, A^A' N. The latter is an example of the alated or 

 winged shells. 



Operculum, or lid. This is only an appendage to the turbinat- 

 ed or spiral shells affixed to the foot of the animal, sometimes of 

 a testaceous, in others of a horny or cartilaginous substance. 

 It acts as a door or lid, and is calculated for the protection of the 

 animal, when it retires within its dwelling, from the intrusion of 

 its enemies, and adapted to the shape of the aperture, which 

 it closes nicely up : as exemplified in the cartilaginous opercu- 

 lum of the common Periwinkle, (the Turbo Littoralis of Linne,) 

 it is of a brown horny appearance ; and is also to be found in 

 the large 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. 12,1) ; and plate III. figures 7 and 8, D D. 



This spire is a prominent feature of the Univalve ; and upon 

 its being elevated, depressed, &c. depends much of the generic 

 and specific definition. Adanson, in his * Natural History of 

 Senegal,' printed at Paris in 1 757, says that the external charac- 

 ter of the spire varies according to the plane they turn upon, 

 which, he observes, is either 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 m.any of the young 

 shells have not the same number of wreaths as the adults ; from 

 which it would appear, that the part of the animal nearest the 

 apex, never increases in size. The number of wreaths cannot, 

 at all times, be depended upon. A full grown shell may. how- 

 ever, be known from the outer-lip, which has generally an un- 

 finished appearance in young shells. Indeed, in all the land and 

 fresh water shells, it is a distinct criterion, as they are never com- 

 plete in the form of the outer-lip, till full grown. 



Whorl is one of the wreaths or volutions of the shell. Plate 

 II. fig. 8,i; and plate III. fig. 10, L. 



Depressed Spire is when the spire is very flat, as in the shells of 

 the genus Planorbis, &c. PI. II. fig. 12, d; and pi. III. fig. 5, s. 



