TH E TASMANIAN NATURALIST. 
2 9 
1. 1 he Tomatellidic , which have a stout, little spiral shell. 
2. The Bull idee. (Fig. 15), in which the spire is concealed. 
3. The Aplysiadae or ‘ Sea hares/ where the shell is flat and 
oblong or triangular in shape. 
The remaining families are slug-like and shell-less. 
CLASS III.—-Pteropoda. These pretty little Mollucs are ocean 
swimmers. 1 'he members of one division to which Cavolina (Fig. 19) 
belongs are furnished with glassy, transparent shells. They may be 
taken alive from the surface by a towing-net; the dead shells very fre¬ 
quently appear in dredgings in deep water. Several species have lately 
been taken at the 100 fathom line off Cape Pillar. 
CLASS IV.—J .amellibranchiata (Plate gilled) or Conchifera 
(shell-bearing) includes the Mollusca, commonly known as 1 bivalves/ 
the animal being snugly hidden between two more or less closely fitting 
shelly valves. 
The oysters, 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 eqmvalve when the two shells composing it are of the same 
size ; inequimtve when they are not. 
1 1 the umbones are in the middle the shell is equilateral (Fig. 27), 
but inequilateral- when they are nearer one side than the other (Fig. 24). 
If the shell bean oyster or a scallop, you will find on the inside a single 
circular, scar-like mark near the centre ; this is the point to which the 
muscles that close the valves, and hold them so tightly together, are 
attached. In the majority of bivalves, however, there are two such mus¬ 
cular impressions or scars, one on either side of each valve of the shell. 
The former group on this account are often called Mommy aria (having 
one shell muscle), and the latter Dimyaria (having two shell muscles). 
In the last-named the two muscular impressions are united by a fine 
groove (or pal Hal line), which in some runs parallel to the margin of 
the shell (Fig. 25), whilst in others it makes a bend in (palUal sinus) on 
one side of the valve towards the centre (Fig. 24). In Monomyaria it 
will be found running parallel to the margin of the shell. It marks the 
line of attachment of the mantle or shell-secreting organ of the animal 
to the shell, which grows by the addition of fresh matter along its edges; 
so that the concentric curved markings so often seen on the exterior 
correspond in their origin with the periodic mouths of the Gastropods. 
The bivalves are all aquatic, and many bury themselves in the sand 
or mud by means of a fleshy, muscular foot. These are furnished with two 
siphons or fleshy tubes, sometimes united, sometimes separate, through 
