THE TASMANIA N X AT U R A LI ST. 
which they respire, drawing the water in through one and expelling it by the 
other. I hose kinds whose habit it is to bury themselves below the sur¬ 
face of the mud or sand are furnished with long, retractile siphons, and 
to admit of their being withdrawn into the shell the mantle is at this 
point attached further back, giving rise to the pallial sinus above 
described ; this sinus is deeper as the siphons are proportionately 
longer, and in many cases, too, the valves do not meet at this point 
when the shell is closed. Attention to these particulars is necessary 
when arranging your bivalves, as on them the classification depends, 
the class being divided into — 
a. Asiphonida (Siphonless). 
b. Siphonida, 1'niegro-palliala (with Siphons) — Pallial line 
entire. 
c. Siphonida, Sinu-paUicila (with Siphons) — Sinus in pallial 
line. 
Division a. —Asiphonida is next sub-divided into— 
1. The Osireidae or Oysters, which are deservedly a distinct 
family in themselves. We have one fine large species. 
2. Anomiadae , comprising the multiform and curiously con¬ 
structed Anemia. 
3. The Pecienidae, taking its name from the genus Pecten or 
4 Scallop-shells,’ of which one kind (P. medius) is frequently seen in 
the fishmongers’ shops ; altogether, we have five species, most of 
which are fine, handsome shells when good specimens are obtained. 
The l '\ horny oysters,’ Spondylus, take rank here, and are highly 
esteemed by collectors ; the genus is represented in our fauna by 
one small species only. 
4. 'The Aviculidae or 4 Wing shells’ (Fig. 31), among which are 
numbered the 4 Pearl oyster ’ of commerce ; and also Pinna , of which 
we possess one fine species. 'The Pinnas, like the mussels and some 
other bivalves, moor themselves to rocks by means of a number of 
threads spun by the foot of the mollusc and termed the dysuts. 
5. The Mj fiiidae or Mussels. 
6. The Arcadae or 4 Ark shells,’ characterised by their long, 
straight hinge line, set with numerous fine teeth (Fig. 22); the genera 
Glycimcris (Fig. 25) Nucula and Leda (Fig. 29) also belong to this 
family. 
7. The Trigoniadae , whose single living genus, Trigonia , is 
confined to the Australian and Tasmanian coast line, whereas in 
times now long past they had a world-wide distribution. 
S. The Unionidae , comprising the fresh-water mussels, of which 
one species inhabits our rivers, but is said to be found only in those 
which flow into Bass Strait. 
