102 On the different Ages o/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. 1 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 saoie, raillims. ; 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 Jiilo- 

 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 Tr. 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 Tr. 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 I'racture; 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 

 purple 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 acciclentally omitted by our 

 correspondent, will be given in a note in our next. — Ed.] 



