15-2 



investigated by Mr. Sowerby*. These shells may be dis- 

 tinguished as multilocular bivalves. 



Univalves vary considerably in their forms, being tubular, 

 discoidal, fusiform, turbinated (the spire passing suddenly 

 from the tumid belly of the shell), or turriculated (the turns 

 gradually enlarging, so as to form an elongated cone) ; they 

 are also ovoid, elliptical, &c. The surface also varies con- 

 siderably, being smooth, striated, trellised, grooved, &c. 

 and is, in some shells, beset with rounded or sharp ribs, 

 spines, foliaceous processes, &c. The shell (PI. ix. fig. 2) 

 terminates superiorly with the top of the spire, which is 

 pointed, obtuse, or globular, and sometimes decollated, as 

 is chiefly observable in some shells of the genus bulijnus. 

 The spire (a) is formed by the union of the turns, or whirls 

 (h), which are counted by reckoning the lower turn, con- 

 taining the opening, see below, as the first, and reckoning on 

 the same line to the top of the spire. The turns generally go 

 in a direction from the right to the left ; they very rarely pass 

 in the opposite direction, and, when that is the case, they are 

 termed reversed shells. The line at which the whirls are 

 united to each other is termed the suture. The whirls are 

 either plain, keeled, grooved, crenulated, or crowned with 

 points, tubercles, or spines, &c. 



The hack (c) is the external tumid part of the last turn 

 which forms the opening; the belly (d) is the corresponding 

 concavity, reaching to the part where the cavity contracts, 

 which is called the throat. The opening (e) is circular, 

 semicircular, oval, angular, &c. and is whole, or terminates 

 in a groove or a notch, which is either straight or reverted : 

 when the opening is longer than wide, it is said to be longi- 

 tudinal ; when wider than long, transverse; and when 

 straight and narrow, linear. The edge, or margin of the 

 opening, is divided into the right and left lips : the right, or 



* Mineral Conchology, Vol. i. p. 154, &c. 



