1849.] SHARPE ON THE SECONDARY ROCKS OF PORTUGAL. 169 



It seems from this analysis that the different depth of water under 

 which the beds were formed has been one principal cause of the dif- 

 ferent proportions in which we find northern species entombed in the 

 Portuguese formations. Yet after making every allowance on that 

 head, there seems to have been a somewhat greater power of migration 

 in the earlier periods, as the known species of Lamelhbranchs are 

 proportionally more numerous in the Jurassic than in the subcretaceous 

 beds, and in these than in the hippurite limestone, and the known 

 Gasteropods of the subcretaceous beds are proportionally almost twice 

 as numerous as those of the hippurite limestone. 



I have dwelt at some length upon these speculations, because I 

 believe that this is the first time that any large collection of secondary 

 fossils from so southern a part of Europe has afforded the opportunity 

 of comparing the inhabitants of the southern seas of the secondary 

 periods with those of the secondary seas of our latitudes. The 

 secondary collections hitherto made in Europe extend over many 

 degrees of longitude, but have been very limited in latitude, and have 

 therefore come from countries with moderate differences of climate. 



I cannot conclude without expressing how much I am indebted to 

 Mr. Morris for the assistance he has given me in the examination of 

 the whole of my collection of fossils, and to Mr. Edward Forbes for 

 his kindness in examining and describing the Echinodermata brought 

 from the beds here described. 



