154. EOCENE MOLLUSCA. 
A great many species in this genus (Astarfe) have the interior margins of the shell 
ornamented with crenulations, while in others these margins are plain. ‘The same 
distinction may be seen in the genus MVucula, where some species have a crenulated 
margin, while others are smooth. There is, however, this difference between the two 
genera—viz., that in Mucula the young shell, as well as the old, is furnished with this 
kind of ornamentation in those species which possess it at any period of their lives; 
but in the case of Astarte it is not so. In the ‘ Crag Mollusca,’ vol. ii, p. 173, I have 
said, that “in all the species of this genus the young state of the shell has its margin 
free from crenulations, and never until it has attamed to maturity does it assume that 
character, and whenever a specimen has its margin crenulated, it may be considered to 
have arrived at its full growth.” Mr. Jeffreys appears to be of a contrary opinion, and 
in his paper entitled “ Additional Gleanings,” published in the ‘ Annals and Mag. Nat. 
Hist.’ for September, 1859, p. 5, says of Ast. triangularis, “ the non-crenation of the margin 
does not depend on age, for I possess specimens which are evidently adult and of the same 
age, some of them having the margin quite plain, while in others it is strongly crenu- 
lated.” Also in ‘ British Conchology,’ vol. i, p. 309, the same opinion is repeated. 
If by ‘evidently adult” it is meant that the full size is attained, I answer that size, 
though in general a good criterion of age, is not so always; there are dwarfs 
and giants in almost every species; and although occasionally one individual which 
has its margin crenulated may be sma//er than another of the same species which 
has it smooth, it does not necessarily follow that the smaller one is an immature shell, 
or that the larger one had completely attained to full maturity. It is very difficult— 
perhaps impossible—to prove it so in this genus, but it bears great probability from an 
analogous case in the genus Cyprea, for example, where the adult state is denoted by an 
alteration of form. I have found specimens of the /v//-grown state of Cyprea Kuropea 
which 7x /inear measure is not more than one third the size of the largest adult of the 
same species, and yet the two extremes may fairly be assumed as full-grown mdividuals, 
indicated by their peculiar characters. I have never been able to find, although I have 
examined thousands of specimens of the genus Astarfe (which is most abundant in the 
Coralline Crag), a single individual w7t/ a crenulated margin which could be assigned to 
the young state of a species which has a margin so ornamented when full grown. 
In the diagnosis of the animal of the genus Astarfe by Messrs. Forbes and Hanley, 
p- 455, vol. i, it is said, “the mantle is freely open m front, p/az at the margins.” The 
same plain condition of the mantle-margin is confirmed by Mr. Jeffreys, ‘ Brit. Conch.,’ 
vol. ii, p. 308. Now, in order to produce the knobs which ornament the margin of the 
valves, I imagine the mantle would be fimbriated or pointed, to enable the animal to 
deposit calcareous matter in that form; and if it be not so inthe young state—as I presume 
is the case—it would become so in the full-grown animal. 
I once thought the fimbriated margin of the mantle might have indicated a sexual 
difference (see ‘Mem. Geol. Surv.,’ vol. i, p. 414, 1846), but I now consider the crenu- 
