102 On the different Ages of Trochus niloticus and T. maximus. 
amount of individual variability. Some young individuals of 
Tr. maximus are as broad as high, some even a little broader 
than high; and, on the contrary, in the younger age of Tr. ni- 
loticus {stage of marmoratus), its breadth exceeds its height by a 
relatively smaller amount than in the full-grown shell. I have 
before me two very young specimens (stage of spinosus), which 
I am induced to regard as niloticus by their relative breadth ; 
the height of both is the same, _millims.; the breadth of the 
one ,of the other  millims.*, which last proves to be a very 
excessive one when compared with those of other young indivi- 
duals. The even and smooth surface of their last whorls is the 
most characteristic feature of the adolescent specimens of nzlo- 
ticus (marmoratus) ; it is the consequence of the disappearance 
of the sculpture long before the change of shape peculiar to the 
full-grown age makes its appearance at the same time as the 
last whorl; whilst in Zr. maximus both changes, which are of 
less intensity, coincide with regard to the age of the individual, 
But even this vanishing of the sculpture in 77. niloticus takes 
place in some individuals a little sooner or later than in others : 
the amount of this variation may be a whole whorl ; and external 
causes seem to have some influence upon it: in fact one of the 
specimens in the stage of marmoratus shows the traces of having 
been fractured just in the whorl, where the change generally 
takes place very gradually; but here the sculpture is preserved 
in its full strength up to the fracture, and immediately after it 
the newly formed continuation is smooth. There is no evi- 
dence that a rather large portion of shell has been destroyed 
and taken away by the fracture; on the contrary, the perfect 
regularity of the following portion of the whorl shows that there 
is no marked restoration, but simply progress of growth ; never- 
theless the change of sculpture is sudden, as if the interruption 
and new beginning had given the animal an impulse to construct 
the following parts of the shell at once according to the new 
fashion, instead of gliding gradually from one into the other. 
Another instance of individual variation is presented by a speci- 
men of niloticus which shows on its last whorl the dilatation 
and swelling of the lower edge which is so very characteristic 
of the last whorl of the full-grown shell, whilst its dimensions 
(height 56, breadth 66 millims.), the still even base, the broad 
urple rays above, and the small spots beneath rather clearly 
indicate that another whorl is still required for the full growth 
of the shell. Such specimens, in which a property normally 
peculiar to the adult makes its appearance in a previous stage of 
growth, may be called premature individuals. 
* (These measurements, which have been accidentally omitted by our 
correspondent, will be given in a note in our next.—Eb. |] 
