152 W. A. KIDD, FSQ., M.D., BS., M.R.C.S., F.Z.8., ON 
advance, which it truly was, on the old style of ships’ 
bottoms, when the great seamen of old days would be 
obliged after a long voyage to spend weeks in “ careening ” 
the bottoms of their stout, rough little vessels that the surface 
might be freed from myriads of barnacles, seaweed, and such 
like. So the growth of knowledge produced copper bottoms, 
and in course of time this process was improved upon by 
the discovery of compositions to preserve the copper 
bottoms themselves. But here in a molluscan shell is this 
late discovery of man anticipated in an organic composition, 
formed from the shell itself, and reformed as required. 
It only remains to remark upon the exceeding beauty of 
the colouring of many shells, connected with the | presence of 
certain olands i in the “mantle” of the mollusc. The coloars 
may be white, red, green, yellow, olive, purple, slate blue, 
black, and marked with a marvellous symmetry. As to the 
forms of shells, there are those of cephalopods, such as cuttle- 
fishes, argonauts, pearly nautilus, octopus, and a few more. 
Ammonites and Belemnites, among extinct forms, come 
under this division. Of these, all the pearly nautilus and 
Argonauta argo, with its shell used as a boat in which the 
molluse swims near the surface, are the most beautiful and 
familiar forms. But the protection conferred on cuttlefish 
is very interesting, with the dorsal plate or cuttle-bone 
placed under the skin of the back so as to protect the animal 
against collisions as it swims backwards, as also the 
remarkable ink-bag which can be discharged by way of self- 
defence against pursuers at the pleasure of the animal. 
Next to Cephalopods come the bivalves, oysters, scallops. 
cockles, mussels, and razor-shells, too familiar to need 
description. 
The largest division is that of Gastropods, such as snails, 
whelks, periwinkles, limpets and cowries, which inhabit fresh 
water ‘lakes, rivers, salt water at all levels and in all regions, 
and the land. 
Some of these have an internal skeleton, but the majority 
have an external skeleton, and some, such as slugs, none at 
all. They have spirally-coiled shells and are univalve as a 
rule, and nearly always they are coiled from night to left. 
To take one familiar example out of many as to efficient 
protection, we may remember the numerous and _ fruitless 
efforts made by us in the days of our youth to dislodge a 
well- grown limpet from its rocky home, and may thus gain 
an idea of the power of the muscle which retains it in 
