50 HANDBOOK OF SHELLS. 



tlegans is a common shell on our chalk-downs, and well illus- 

 trates its family, in which the mouth is nearly circular. Foreign 

 examples of this genus are much esteemed by collectors. The 

 other two families are, (2) Helicinida and (3) Aciadidcc. 



Order III. OPISTHOBRANCHIATA. These animals carry 

 their gills exposed on the back and sides, towards the rear of 

 the body. Only a few have any shell. I. The Tornatellidcc^ 

 which have a stout little spiral shell. 2. The 

 Bullida, in which the spire is concealed (Fig. 

 26). 3. The Aplysiada, where the shell is flat 

 and oblong or triangular in shape. The re- 

 maining families are slug-like and shell-less. 



Order IV. NUCLEOBRANCHIATA. Derives 

 its name from the fact that the animals consti- 

 tuting it have their respiratory and digestive 

 organs in a sort of nucleus on the posterior 

 part of the back, and covered by a minute 

 Fig. ^i^Bulla shell- As they are pelagic, the shells are not 

 ampulla. readily to be obtained. They are divided into 

 two families, Firolida and Atlantidce. 



CLASS IIL PTEROPODA. Like the last, these pretty 

 little mollusca are ocean-swimmers. The members of one 

 division of them, to which the Cleodora belongs, is furnished 

 with iridescent external shells. 



CLASS IV. The LAMELLIBRANCHIATA (Plate- 

 gilled), or CONCHIFERA (Shell-bearing), includes the mollusca 

 commonly known as " bivalves," the animal being snugly hidden 

 between two more or less closely fitting shelly valves. The oys- 

 ters, cockles, etc, are examples of this class. The two valves are 

 fastened together near their points, or beaks (technically called 

 umbones), by a tough elastic ligament, sometimes supplemented 

 by an internal cartilage. If this be severed and the valves parted, 

 it will be found that in most cases they are further articulated by 

 projecting ridges or points called the teeth, which, when the 

 valves are together, interlock and form a hinge ; the margin of 

 the shell on which the teeth and ligament are situated is termed 

 the hinge-line. A bivalve is said to be equivalve when the two 

 shells composing it are of the same size, ineqnivalve when they 

 are not. If the umbones are in the middle, the shell is equila- 

 terat (Fig. 15) ; but ineqtiilateral when they are nearer one side 

 than the other (Fig. 16). If the shell be an oyster or a scallop, 

 you will find on the inside a single circular scar-like mark near the 



