MONOGRAPH OF THE GENUS STROPHIA. 
147 
collected twelve, which were all clinging to some low bushes, but 
although I spent several hours in two days and examined every branch 
and twig of every bush in the vicinity, and searched all other portions 
of the key, I could not discover another living specimen of Strophia 
eburnia. 
I was therefore thoroughly satisfied that I had chanced upon a 
second species, S. albea being the other, which was on the verge of 
extinction. The large number of dead shells, some of which showed 
marks of having been lying on the sand for many years, proved most 
clearly that individuals of this species were abundant once, and that 
the last stronghold of the species was the bushes on the borders of the 
little hill. The species had fought bravely for an existence against 
some foe which it could not conquer, and had gradually succumbed, 
leaving the battle-field strewn with its dead, while I was just in time 
to gather the handful of survivors, among which, I ought to mention, 
was a single young specimen. 
Now what was the foe that proved so fatal to this species? From 
a long and careful consideration of the subject, aided by many facts 
which I cannot now mention, I unhesitatingly answer, the sand, on 
which the dead shells now lie strewn. Once, in all probability, this 
tract was naked rock, covered with palms or bushes. Then the 
Ivory Strophia found a congenial home there, for during wet weather it 
would pass from bush to bush, or from palm to palm, and thus joining 
its fellows, on neighboring bushes, by mutual cross-fertilization increase 
the race. Changing ocean currents formed a sand beach outside the 
rocks and this sand was carried by the winds over the narrow neck of 
land, in time completetly covering the rocks. Now, even in wet 
weather, the Strophias cannot well travel on sand, thus, when each 
bush or palm was surrounded, the mollusks on it became as much 
isolated as would be a ship-wrecked mariner when cast upon a lonely 
island, miles from other land. Then two things occurred, gradually 
the wind, especially during hurricanes, threw the Strophias off their 
feeding grounds on to the sands, from which they could not escape, 
hence they perished. The decay of the race was probably hastened 
by the fact that when a few were left, they became, in time, too closely 
related to perfect fertilization, thus after a time few or no young 
appeared. Hence I found only a single young specimen among a 
dozen adults. 
How long it took for the sand to cover the neck of land is im- 
possible to say, but, after the beach was formed, it was probably not 
