70 Conchology. 



ing a sinus from the aperture throughout the whole length of 

 the beak, fig. 3, o. 



Umbilicus is the opening or perforation in the lower part 

 of the body, or first whorl of many spiral univalves, and is 

 very conspicuous in a number of the trochus and nerita genus 

 in particular. This umbilical preforation runs in a straight 

 line from the base to the summit of the shell, forming through- 

 out a spiral groove or gutter, which is wide at the entrance 

 and tapers gradually towards the apex. This opening occurs 

 in many shells at the base of the pillar, fig. 2, u. A shell with- 

 out an umbilicus is termed imperforate ; sometimes the term 

 imperforate implies that it has neither umbilicus, nor canali- 

 culation at the base. 



Operculum is a testaceous or cartilaginous appendage, pe- 

 culiar in a considerable degree to the univalve tribe of shells, 

 and those only of the spiral or turbinated kinds. This appen- 

 dage is not connected with the shell, but the animal ; and serves 

 like a lid or little door, to protect or close up the aperture of 

 the chamber when the creature retires within its habitation. 



BtVALVES. These consist of two valves united by means of 

 a cartilage, hinge, connection of the teeth or other process. 

 In order to constitute a bivalve shell, it is only requisite that 

 it be furnished with two connected valves without regard to 

 their resemblance in form or dimensions. Some of the bivalves 

 have both valves formed alike ; in others they differ only in a 

 slight degree, and again in others they are altogether dissimi- 

 lar. Shells having both valves alike are called equivalves. 

 Equilateral valves imply those which have both sides of the 

 same valve alike, as for instance, when a longitudinal line is 

 drawn from the beak to the opposite margin, the space on 

 each side of the line is distinguished by the appellation of the 

 right and left side ; and when the form of both of those spaces 

 correspond, the shell is equilateral : the inequilateral valves 

 are the reverse of this, a line drawn as above described, from 

 the beak to the opposite margin, presenting two sides of a 

 very different shape. Sub-equilateral shells, are those having 

 the valves nearly equal at both sides. 



All bivalve shells do not completely close their shells ; while 

 others, when the shells are shut as closely as their form will 

 allow, still exhibit a kind of hiatus, or gaping, either at the 

 anterior or posterior end, or at both; and in some, when the 

 valves are shut, both the anterior and posterior parts are 

 closed, but an opening appears on one side of the beak. 



One of the first circumstances to be considered is, which 

 part of a bivalve shell ought to be deemed the base, because 



