THE NAUTILUS. 91 



Long. 2.4, alt. 2.2, diam. 1.7 mill. 



Hab. : Lilycash creek, Joliet, 111. Probably it has been over- 

 looked in materials from other places, owing to its resemblance to 

 immature specimens of some forms of P. compression Pr. When 

 once known it will always be recognized. It also resembles some 

 forms of P. paupercidum Sterki in size and shape, but its compara- 

 tively coarse striation will distinguish it at once. Pis. handwerki is 

 not a showy Pisidium, with striking features, but nevertheless a 

 good species. 



New Philadelphia, 0., Nov., 1899. 



ORIGIN OF THE MUTATIONS OF OSTREA.' 



The oysters are a proverbially difficult group, owing partly to their 

 adherent situs and partly to the fact that they have not hitherto been 

 studied with regard to the direct influence of the environment on 

 individual specimens. That this is very great I have convinced 

 myself from a prolonged study of a multitude of specimens of 0. 

 virginica of which the provenance was known, and of many hundred 

 specimens of our tertiary species, which usually show from the 

 character of the scar of attachment something of the circumstances 

 in which they grew. The conclusions to which I have been led by 

 this study may be regarded as in part provisional, but in the main 

 highly probable, and as furnishing a first contribution to the sort of 

 study which is essential if we would understand the processes of nature 

 through which these animals acquire their most conspicuous external 

 characters. They may be regarded as especially applicable to the 

 Crassostrea group. 



Leaving out of account the nepionic characters, the characteristics 

 of the adult shell may be summarized and derived as follows: The 

 most permanent characters of the shell, and the best, if not infallible 

 guide to specific recognition among the puzzling mutations a largo 

 series presents, are the form of the hinge-margin, the minute sculp- 



] This interesting extract by Dr. Wm. H. Dall is taken from bis review of the 

 Tertiary Osireidse in the Transactions of the Wagner Free Institute of Science, 

 Vol. Ill, Part IV, p. 675, 1898. As the original paper deals chiefly with ter- 

 tiary forms, and is probably not accessible to many interested in recent oysters, 

 we reprint it here. 



