WHITE.] CONCLUDING EEMAKKS. 27i 



the Pacific coast ; and yet, for reasons snggested in the follovring re- 

 marks, it is probable we shall be able to correlate the strata of the Upper 

 Missouri River region with those of the Texas region more directly than with 

 those of the Pacific coast. No doubt the Cretaceous sea connected the 

 Texas aud Upper Missouri Eiver regions directly, without the interposi- 

 tion of land surface, great or small, and the strata of Cretaceous age may 

 now be traced nearly or quite continuously from one region to the other ; 

 while it is certain that one or more continental factors were interposed 

 between these, together with their intermediate region, and the waters 

 which then covered what is now the Pacific coast region. It is in the in- 

 termediate region between that of Texas and the Upper Missouri Eiver 

 that my labors for 1877 were prosecuted, and a part of the collections 

 made by others and recorded in the immediately preceding lists were 

 obtained in the same intermediate region. Besides these, collections 

 have been made by various parties of the Uni4;ed States surveys in the 

 same and adjacent regions, all of which show a commingling of the forms 

 which are among those that are respectively pecuhar to each of the sep- 

 arated regions. It is expected that this subject will receive especial 

 attention iu future reports, but the following prominent facts may be 

 noted here, the comparisons being mostly between portions of the Texas 

 and Upper IMissouri River regions, respectively. 



Collections fi'om the Cretaceous rocks of Texas show a remarkable 

 profusion of the Ostreidse, especially of the genera Gryphcca and Exogyra. 

 So far as I am aware, no example of Gryphcca has been found iu any 

 Cretaceous strata of the West north of latitude 41° ; nor any example of 

 Exogyra north of latitude 38° ; these parallels being used for conven- 

 ience as approximate boundaries. While it is possible they do exist north 

 of those boundaries respectively, it is very certain they are exceedingly 

 rare there. These remarks apply only to Cretaceous rocks, for GryjyJicca 

 is well known to exist in Jurassic rocks much farther north. Southward 

 from those boundaries respectively, the two genera named are well rep- 

 resented, but they apparently reach their gTeatest abundance in Texas, 

 in the rocks of which region at least six species of Exogyra are found. 



Again, the Hippuritidfe are common in the Cretaceous rocks of Texas, 

 but I am not aware that an examxjle of any species of the family has ever 

 been found north of latitude 35° ; and the same may be said of the genus 

 N^cr.iuoja, two or three species of which are found in Texas, but none far- 

 ther northward. Echinoderms are very rare in the Cretaceous rocks of 

 North America, except in those of Texas, where they are not uncommon, 

 but hitherto none but the Echinoidea have been discovered, so far as I am 

 aware. Northward from that region only one representative of the 

 Crinoidea, one of the Asteroidea, and one of the Echinoidea have been 

 found. Besides these difierences, there are many genera and some fam- 

 ilies well represented in one region that are not known or only slightly 

 represented in the other. 



Aside from these faunal diiferences between different regions of North 

 America, which seem not to have been due to separation by then exist- 

 ing land barriers, nor to mere local conditions, there are some regional 

 differences that do seem to have been due to local conditions connected 

 with ijroximity of extended coasts ; such, for example, as those which 

 appear in the fauna of the Coalville series of Cretaceous strata. Further- 

 more, the fauna of all these regions, taken collectively, compared with 

 rocks of admitted equivalency of age iu other i)arts of the Avorld, present 

 differences that cannot be accounted for by any of the causes suggested. 

 These differences are no doubt due in part to the more or less remote 

 effects of conditions which governed the distribution of species, but 



