WAED.] DISCUSSION OF THE TABLK. 523 



genera, by wbicb term we do not here mean either that they are extinct 

 genera, or that they do not occur in higher strata (e. <j., Miocene), or 

 in lower (e. </., Cenomanian), or that they are wholly excluded from 

 either of the three formations, but simply that they predominate in some 

 one relatively to the other two. 



As already stated, the whole number of genera represented in the 

 three formations is -'{ul. Of these, 32 are confined exclusively (so far 

 as these formations are concerned) to the Laramie grouj), 02 to the Se- 

 nouian, and 155 to the Eocene; 49 are common to all three formations, 

 6 are found in the Laramie and Senonian, but not in the Eocene, 23 are 

 found in the Laramie and Eocene and not in the Senonian, and 27 are 

 absent from the Laramie and found in both the other formations. The 

 number found at only one horizon is therefore 249, the number occur- 

 ring at two horizons 50, and the number at all three 49. The number 

 ranging from the Senonian to the Eocene, and therefore, regardless of 

 the Laramie, certainly belonging to both Mesozoic and Cenozoic time, 

 is 70. 



The discussion of the genera may be conveniently separated into two 

 parts, one of which shall be devoted to the consideration of the evi- 

 dence in favor of synchronism, and the other to the subject of geograph- 

 ical distribution. The first of these subdivisions will have nothing to 

 do with any of those genera which are, in the sense here employed, 

 characteristic of any one of the three formations, but must be confined to 

 those that are common to two or all three. Such genera, moreover, as 

 are nearly equally represented in each of the three formations can have 

 no weight in establishing the affinity of the Laramie with the one rather 

 than the other, and must also be excluded from our primary compari- 

 sons. A further exclusion must be made of those genera which are 

 common to the Senonian and the Eocene but absent from the Laramie, 

 since both these formations are treated as known quantities, and com- 

 parison of their common elements could lead to no new results. We are 

 therefore really reduced to such genera as are either confined to the 

 Laramie and Senonian or to the Laramie and Eocene, or are so nearly 

 thus confined as to be fairly characteristic of the two. In deciding such 

 cases we may also properly exclude very small genera, such for instance 

 as are represented by only one or two species in each formation, unless 

 these species be specially diagnostic or very abundant ; but we must 

 not at any time lose sight of the fact that it requires about two and a 

 half species in the Eocene to have the same weight as one in either of 

 the other formations. 



After carefully scanning the table, I have selected such genera as I 

 think fairly illustrate this point, and they may be set down in their 



