266 [November, 



some parts of the world, we have yet only seen from this region, a single frag- 

 ment, too imperfect to give any clue to its generic relations. The paucity of 

 some, and entire absence of others, of the more common genera of Mollusca, 

 such as Ostrea, Oryphcea, Exogyra^ Sfc, in these collections is worthy of notice. 

 Future investigations, it is true, may add more species to our present meager 

 list of these rare forms, yet it is probable we have here something like an ex- 

 pression of the numerical proportions in which many of the lower types of life 

 existed in these ancient seas. 



Of the one hundred and ninety-one species enumerated in this catalogue, 

 forty-four belong to the Tertiary system, and one hundred and forty-seven to 

 the Cretaceous. None of the former are known to occur in the States, or on 

 the other side of the Atlantic, while of the Cretaceous species, nine appear to 

 be common to the Nebraska formations and those of the States, and four are 

 identical with forms occurring in the old world.* Of these nine species hav- 

 ing so great a geographical range, six, or nearly one-third of all that class of 

 Mollusca contained in the list, belong to the Cephalopoda, while nearly all the re- 

 maining one hundred and seventy-six species, which appear to be restricted to the 

 north west, belong to the Lamellihranchiata and Gasteropoda. This, however, is 

 not so surprising when we bear in mind the fact that the habits and organiza- 

 tion of these ancient Blolluscavnust have been such, from what we know of their 

 existing analogues in our present seas, that the former depended on accident, 

 or feeble locomotive organs, for their gradual distribution over the world from 

 their various centers of creation, while the Cephalopoda, owing to their superior 

 locomotive powers, were capable of wandering freely far out over the most 

 profound parts of the ocean. 



In order to convey a clear idea of the relative positions and vertical range of 

 the fossils enumerated in this catalogue, as well as of the nature and order of 

 succession of the various strata from which they were obtained, we give at the 

 end of these remarks, a more com^jlete section of the Nebraska formations than 

 has been yet published. Future investigations may, it is true, show a more 

 intimate relation between the sub-divisions represented in this section than we 

 are aware of at this time, yet so far as we now know, each one of them appears 

 to have been formed during the prevalence of physical conditions so widely dif- 

 ferent from those in existence at the time of the deposition of each of the others, 

 that not a single species of the fossils found in them seems to be common to 

 any two, excepting No. 4 and 5, and perhaps No. 2 and 3, We think, however, 

 when more is known in regard to the range of species in these rocks, it may be 

 found convenient to group them together as follows : 

 Group A. Miocene. 



" B. Eocene. 



" C. No. 4 and 5 of the section. 



" D. No. 2 and 3 " " 



" E. No. 1 " " 



At any rate, by such a classification, we would have divisions which, so far as 

 our present knowledge extends, appear to be characterized, not merely by dis- 

 tinct species, but by rather different types of fossils. 



In one of our former papers, we mentioned the fact that some of the forms in ' 

 division No. 4 resemble those of the upper Green Sand of English geologists. 

 Although this is the case, later collections appear to show that many species we 

 had supposed restricted to the upper of these two rocks, also occur in the lower. 

 In addition to this, we have more recently ascertained that Pleurotoma minor 

 (Evans and Shumard's unpublished MSS.,) as well as a species of Fasciolarmf 



* It is quite probable a few other species may yet be found in Nebraska that 

 will prove to be common to the Cretaceous rocks of that region and those of the 

 States, as well as with those of the old world. We look for them chiefly 

 amongst the Cephalopoda. 



t These genera are generally regarded as having been introduced towards the 

 close of the Cretaceous epoch. 



