iv. 
the shells of Crustacea. There is abundant proof of this destruc- 
tive work on every shore, by the perforated shells strewn about 
in every direction. There are traces of organs of hearing in 
many of the Mollusca, consisting of a calcareous concretion, 
termed an otolith, which is in continual vibration. Among 
the Cephalopods this sac is placed in the cephalic region, 
foreshadowing the auditory arrangement of the vertebrates. 
Ocellary spots occur on the margin of the mantle of Pectens and 
other free-swimming Bivalves. Many of the Gastropoda have 
no eyes ; when present they are small in proportion to the size of 
the body. In the more highly organised Cephalopods they are 
much larger and more highly developed, a necessary provision for 
their active habits. 
Marine Mollusca are mentioned in these pages as inhabiting depths 
of so many fathoms, or frequenting different zones. The phrase 
must be understood to refer to the several belts stretching out in suc- 
cession from the shore to the farthest limits of measurement. These 
have been divided into five zones by the late Professor E. Forbes. 
1. The Littoral zone, between tide-marks. 2. The Laminarian zone, so 
called from the prevailing sea-weed, extends from low-water-mark 
to 15 fathoms. 3. The Coralline zone, extending from 15 to 
50 fathoms; here many of our rarest species are found. 4. The 
ceep-sea Coral zone, extending from 50 to 300 fathoms. Beyond 
this depth is the Abyssal zone, which Forbes considered to be 
scarcely developed within the British area. It must be remarked 
these zones have been much criticised of late. Of all the land and 
fresh-water mollusca Helix and Unio have the widest distribution. 
Some no doubt have been transported great distances by human 
agencies. Cryptomphalus aspersa to Algeria, the Azores, and Brazil, 
and many other places. Tachea hortensis has been naturalised in 
New England, and on the banks of the St. Lawrence. The minute 
Vallonia pulchella is found in Madeira, the Caucasus, and in nearly 
all the northern parts of North America. 
The partial absence of limestone among the Cambrian rocks 
affected the shell-bearing Mollusca, as compared to the succeeding 
