114 THE AUSTRALIAN NATURALIST, 
forms—the anterior, or head valve, semicircular in form with 
elevated apex; the six. median valves, all bilobed and similar 
in shape; and the posterior, or tail valve, which is like the 
median valves, with the addition of a sloping surface behind 
the apex or mucro. 
The surface of the valves of most Chitons is divided into more 
or less clearly-defined areas, adorned with the marvellously 
diversified sculpture forming the principal characters by 
which the various species may be determined. Hach side of 
‘the median valves is generally divided by a diagonal line into 
what are termed the lateral and central areas, and a dorsal or 
jugal tract extends along the ridge of the valve. Asa rule 
the anterior valve is sculptured all over like the lateral areas. 
The posterior valve is divided into a central and a posterior 
area. The interior of the valves also offers distinctive char- 
acters for the determination of species in the projecting plates, 
called insertion plates, to which the girdle is attached. These 
are commonly cut into teeth by transverse slits, varying in 
number with the different species. 
The girdle is variously clothed with scales, spines, haus or 
spicules, varying greatly in development among closely allied 
species. 
So far as the Polyplacophora found on the New South 
Wales coast are concerned, the following genera are rep- 
resented :—Ohiton, Ischnochiton, Callochiton,  Callistochiton, 
Onithochiton, Tonicia, Lorica, Loricella, Plaxiphora, Liolophura, 
Acanthochites, Cryptoplax, Lepidopleurus. 
These are given in inverse order of devyelopment—the 
highest forms first. 
They are found at different stations or depths below high- 
water mark. Some species cling to the outer surface of the 
rocks, others affect the under surface of large boulders, stones, 
or pebbles, while others cluster along the edge of insertion 
where the stones are half buried in sand. 
To the first class belong Chiton pellisserpentis, Plaaiphora 
petholata, Liolophwra gaimardi and Onithochiton rugulosus. 
These four species, owing doubtless to their exposed stations, 
and the fact that they lie between high-water mark and_half- 
tide mark, are the commonest and most frequently found 
in collections. 
In shallow water below low-water mark, and in the deep 
rock pools, most of the other species are found, the numerous 
and variously coloured Ischnochitons being generally within 
reach of the ordinary wader who is too lazy or has not 
the proper equipment for deeper water searching. Here also 
are found the handsome Chiton jugosus, Onithochiton quercinus, 
Callochiton platessa, and Callistochiton antiquus with its remark- 
able sculpture. 
