210 lloyctl Society, 



capsule containing a fluid; while in the minute ovum of Mammalia 

 there are found all the essential elements of the egg of birds and other 

 Ovipara, namely, an external membrane, analogous to the vitellary 

 membrane, but performing a different function; a granular membrane, 

 containing a thin fluid, corresponding to the immature yelk of a bird's 

 egg ; and a vesicle in every respect analogous to the vesicle which 

 Purkinje found in the hen's egg, while still lodged in the ovary. The 

 author considers the granular membrane, proligerous disc, and gra- 

 nular fluid of the Graafian vesicle, as parts which are superadded, and 

 of which there is no trace within the capsule of the ovary of a bird. 



" Some Remarks on the difficulty of distinguishing certain Genera 

 of Shells ; and on some Anomalies observed in the Habitations of cer- 

 tain Species of Mollusca." By John Edward Gray, Esq., F.R.S. 



In opposition to the opinion of those geologists who consider all 

 shells of the same form and character as having been inhabited by one 

 genus of animals ; that all the species of a genus live in similar situa- 

 tions ; and that all the species of fossil shells, appearing from their cha- 

 racter to belong to some recent genus, have been formed by animals 

 which in their living state had the same habits as the most commonly 

 observed species of that genus, — the author proposes to show, first, that 

 shells having the appearance of belonging to the same natural genus 

 are sometimes inhabited by very different animals ; and, secondly, 

 that some species of shell-bearing molluscous animals live in dif- 

 ferent situations from the majority of the species of the genus to which 

 they belong, or even have the faculty of living in several different si- 

 tuations. Thus, although the animals inhabiting the shells belonging 

 to the genera Patella and Lottia are extremely dissimilar in many 

 essential features of their organization, the shells they form cannot be 

 distinguished from one another by any known character. In other 

 instances, when the animals are very different, the distinctive cha- 

 racters of the respective shells belonging to them are so slight as to 

 be insufficient for the purpose of classing them under separate spe- 

 cies ; and this difficulty of discrimination must be much increased in 

 the cases of fossil shells, especially of those which have no strictly 

 analogous forms among recent shells. 



In support of the position advanced in the second part of the paper, 

 namely, that numerous exceptions occur to the identity of habitation 

 among all the species of the same genus of conchiferous Mollusca, the 

 author adduces examples : 1st, where the species of a genus are found 

 in more than one situation, as on land, in fresh and in salt water; 

 2ndly, where one or more species of a genus, the species of which 

 generally live in fresh water, are found in salt or in saltish water; 

 3rdly, where one or more species of a genus, which is generally found 

 in the sea, are, on the contrary, found in fresh water; and, 4thly, where 

 the same species of shell is found in salt and in fresh water. 



" On the supposed Existence of Metamorphoses in the Crustacea." 

 By J. O. Westwood, Esq., F.L.S., and Secretary to the Entomolo- 

 gical Society. Communicated by J. G. Children, Esq., Sec. R.S. 



The author refers the principal modifications of form which occur 

 during the progressive development of animals to the three following 



