154 EOCENE MOLLUSCA. 



A great many species in this genus {Astarte) have the interior margins of the shell 

 ornamented with crennlations, while in others these margins are plain. The same 

 distinction may be seen in the genus Nucula, where some species have a crenulated 

 margin, while others are smooth. There is, however, this difference between the two 

 genera — viz., that in Nucula 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 attained 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 o{ 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. ii, 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 smaller 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 Cyprcea, for example, where the adult state is denoted by an 

 alteration of form. I have found specimens of the fidl-groicn state of Cyi^rcea Europaa 

 which in linear 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 individuals, 

 indicated by their peculiar characters. I have never been able to find, although I have 

 examined thousands of specimens of the genus Astarte (which is most abundant in the 

 Coralline Crag), a single individual loith a crenulated margin which could be assigned to 

 the youncj state of a species which has a margin so ornamented when full grown. 



In the diagnosis of the animal of the genus Astarte by Messrs. Forbes and Hanley, 

 p. 455, vol. i, it is said, " the mantle is freely open in front, plain 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 in the 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- 



