210 On a Monstrous Oysier-ShelL 



the Poecilia surinamensis, Val., in the ^Anuales des Sciences 

 Naturelles/ 3rd series, vol. i. p. 313. plate 17, to which my own 

 observations, except with reference to the earlier changes of the 

 embryo, will add comparatively little, when published. That the 

 fish observed by Dr. Dowler is the same as that I had an oppor- 

 tunity of investigating, his description shows very plainly. 

 There is only one fact to which I would again call attention, 

 though I have already noticed it before, that the genus Mollie- 

 nesia of Lesueur is founded upon the male of the same species 

 he has described as Pcecilia multilineata. There cannot be the 

 slightest doubt about it, for I have repeatedly seen them copulate ; 

 and among a large number of specimens examined, all those 

 that answer to the description of Mollienesia latipinna are males, 

 and all those corresponding to the description of Pwcilia multi- 

 lineata are females. There are several species of this family 

 much smaller than this Pcecilia multilineata ; indeed, it contains 

 the smallest representatives of the great type of Vertebrates. 

 My Heterandria formosa, for instance, when full-grown, is not 

 quite an inch long, and does not weigh more than five grains. 

 An adult male weighed 33^ milligrammes. 



L. Agassiz. 

 Cambridge, U. S. Aug. 22, 1854. ^ 



XIX. — On the anomalous Oijster- Shell described in the ^Annals' 

 for February. By Dr. J. E. Gray. 



To the Editor's of the Annals of Natural History. 

 Gentlemen, 



I HAVE received an explanation of the anomalous Oyster-shell 

 described by me in your last Number, from my friend Dr. Gray, 

 and as it appears to me wholly satisfactoiy, I forward his note, 

 for the benefit of those of your readers who, like myself, may 

 not have been aware that similar monstrosities, as I am informed; 

 by him, are by no means of unfrequent occurrence. 



Your obedient servant, .> 



George Busk, j, 



"My dear Busk, ' 



" I have little doubt the shell you described in the last Number 

 of the ^Annals' is that of an Oyster [Oslrea edulis), growing on 

 the inside of a valve of Pholas Candida. The inside of the shell 

 of that species has markings corresponding to the tubercles and 

 lines on the outer surface, and in the specimen figured these 

 markings are impressed on the outer surface of the Oyster-shell. 

 It further appears, that the shell of the Pholas must have been 

 that of a dead specimen, since it had growing upon it a Mem- 



