153 



outer lip (f), reaches from that point where it rests on the 

 last turn but one of the spire to the base of the shelL This 

 Hp is notched at its base in the buccinums, and channelled 

 or grooved in the niurexes, and has a fissure or notch in its 

 side in pleurotomas. In several shells, as in the helixes, it 

 extends over to the left side ; in some shells the right lip is 

 earedy having an appendix, differing in different species ; 

 being straight, smooth, digitated, &:c. The left Up (g ) is 

 opposite, of course, to the right, being that part which is 

 attached to the second turn of the spire, and is but of small 

 extent in those shells the opening of which is entire. The 

 columella (h)^ the little column round which the turns are 

 formed, is on the left side of the shell, and terminates at its 

 base ; sometimes in a point, and sometimes in a hollow, 

 which is termed an umbilicus : the surface of the columella 

 varies considerably in different genera, being smooth or flat, 

 or rounded, and sometimes plaited, transversely or ob- 

 liquely ; and sometimes dentated, channelled, tuberculated, 

 &:c. : in some instances the umbilical termination is filled, as 

 if by an exudation of callus. The opening is sometimes 

 filled by a testaceous, or, sometimes, a cartilaginous body of 

 a convex external surface, but fiat on the side next the 

 animal, where it is marked with a spiral line ; this is termed 

 the operculum. 



Bivalve shells, when their valves are similar in size and 

 form, are said to be equivalved, if not, inequivalved ; when 

 the anterior part agrees in form and size with the posterior, 

 they are said to be equilateral^ if not, inequilateral. The 

 valves are connected at their base by a ligament^ with or 

 without a hinge, this ligament being placed either externally 

 or internally : the belly ^ PI. ix. fig. 8 (a)^ is the most tumid 

 part ; the disk (h) is that part between the belly and the 

 margin (c), which is considered to refer to the external side, 

 or, as it may be termed, when the shell is placed on its 

 base, the upper side ; then the eminences^ (umbones) (</), 



x. 



