VARIATION IN SPACE IN GEOLOGICAL TIMES 145 



of the Gryphcea arc^ta, a very characteristic 

 species of the lower Lias or Sinemurian stage. In 

 the Paris Basin, and as far as North Burgundy, 

 this oyster presents itself in its classical aspect, 

 with, that is to say, a narrow and deep shape, a 

 large valve with a very big and very incurved 

 top on the opercular valve, with a very pronounced 

 furrow on one of the sides. As soon as we advance 

 to the south, towards the Macon and Lyons districts, 

 the beds of gryphcece, then contain only rarely this 

 northern form and we find predominating a race 

 with a wider and shallower shell, with the top of 

 the great valve thinner and less curved, and a 

 fainter lateral furrow. This form, sometimes desig- 

 nated by the name of Gryphcea striata, constitutes 

 a real southern or Rhodanian race of the same 

 species. 



In the Tertiary age the facts of the regional 

 localization of certain forms have been better 

 brought to light, amongst others, by Sacco for 

 the Tertiary Molluscs of the valley of the Po, and 

 by Fontannes for the Pliocene Molluscs of the valley 

 of the Rhone and of Roussillon. This last scholar, 

 of whom I have pointed out above the ultra- 

 sectionist tendencies as regards the Ammonites of 

 Crussol, has since become wiser by experience and 

 much more cautious in the creation of new species. 

 Out of three hundred and fifteen species, of which 

 is composed the fauna of the Molluscs of the Pliocene 

 gulf of the Valley of the Rhone, eighty are described 

 as simple varieties of the species known in the Italian 

 deposits, and Fontannes brings out the regional 

 character of these varieties by such names as rho- 



Iv 



