THE EUROPEAN SEAS. 



245 



existence of their present representatives. As we 

 recede in time, the guidance of this generic distri- 

 bution becomes safer and more available to the 

 palaeontologist. 



The zone of the maximum of a genus is that in 

 which it exhibits its greatest number of specific 

 forms. In the iEgean, Cardium has its maximum 

 between 20 and 35 fathoms deep, where it is 

 represented by six species ; Pecten, at between 60 

 and 80 fathoms, w T here it has eleven. In both 

 cases, the zones in which these genera are most fully 

 represented, numerically, are very different ones : 

 all the species of Cardium put together do not 

 amount to the individuals of the single Cardium 

 edide, which occur within the first 12 feet from the 

 margin ; so, also, with Pecten opercularis at a some- 

 what greater depth. 



The Bissow, as might be expected from their 

 habits and food, have their maximum in the sub- 

 littoral zone ; there, also, they swarm numerically. 

 Whenever these two conditions are combined, the 

 palaeontologist has a safe indication of marginal sea- 

 bed. This genus has its deep-water representative, 

 however. Trochus has its maximum in the iEgean 

 in depths between 10 and 20 fathoms; but the 

 excess in this case is very small, and the genus 

 may be said to be fully represented from the mar- 

 ginal line down to 100 fathoms; generically, 

 therefore, this genus is not very characteristic of 

 definite depth. The Pleurotomce have their maxi- 



