Association of American Geologists and Naturalists. 253 



the yellow sand are species common in the lower or greensand division, 

 Oslrea falcata being one, may not some doubt exist, I would suggest, as 

 to the expediency of placing the intermediate straw-colored limestone so 

 definitely on the parallel of the chalk of Europe ? Influenced by the 

 fact just mentioned, and the still inore vveighty consideration that the 

 American strata in a list of about one hundred and sixty organic forms 

 contain probably not more than six or eight species in common with the 

 cretaceous rocks of Europe, I may be allowed to repeat a suggestion 

 made in my Report on the geology of New Jersey, that a further com- 

 parison of the organic remains is required, before we can determine 

 more than approximately the degree of affinity between the several di- 

 visions of the cretaceous series of the two continents. Mr. Lyell, who 

 when in this country collected, with Mr. Conrad's assistance, a some- 

 what extensive group of fossils from the straw-colored limestone of New 

 Jersey, will probably soon give us a more ample insight into the exact 

 degree of affinity subsisting between this stratum and the chalk. 



According to Dr. Morton, the shells hitherto ascertained to be com- 

 mon to the cretaceous deposits of Europe and America, are four : Tri- 

 gonia alceformis, Pecten quinquicostatus, Ostrea falcata and Gryphea 

 vomer ; and to these links he adds about four species of fishes, and that 

 strange gigantic oceanic lizard, the Mososaurus. To these points of 

 agreement we must add those of mere analogy in the remarkable generic 

 affinity of the fossils of the two distinct cretaceous basins. But even in 

 some of the more positive links above named, we recognize in the dis- 

 crepancies of their position in the two series of deposits, the difficulty of 

 establishing an exact equivalency between the strata of basins originally 

 unconnected. Thus, while the Pecten quinquicostatus of our greensand 

 or lower cretaceous group is absent from the middle and upper divisions, 

 which have been placed on the same horizon of time with the chalk of 

 Europe, it occurs on that side of the Atlantic in all the strata of the 

 series ; and again, the Ostrea falcata, restricted 1 believe in Europe to 

 the limits of the chalk or upper creiaceous group, abounds in this coun- 

 try chiefly in the lowest and disappears in the middle. In these in- 

 stances we see exemplified a general and important law concerning the 

 distribution of fossils, which is, that those species whose geographical 

 distribution is the widest, possess likewise the greatest vertical range, or, 

 adapted to a greater variety of localities and physical conditions, they 

 have been suited to withstand a greater series of vicissitude, and to en- 

 dure therefore a longer time. Such, though usually styled the charac- 

 teristic fossils, are in reality the least characteristic of all ; for, while 



Vol. y.mn, No. 2.— July-Sept. 1844. 33 



