wiirrE] GENERAL DISCUSSION. 251 



Eiver Group ; and also by the fact tliat several similar fossHiferons hori- 

 zons occur near to and both above and below the plane where, for stra- 

 tigTaphical reasons, we And it desirable to make the division between 

 the two groups. 



Among- some of the better known molluscan species just referred to, I 

 found in these debatable strata, about three miles east of Table Kock 

 Station, a small undescribed Planorhis, which I have called P. cirrus.* 



The last field-work that was done for the season was an examination 

 of the Laramie strata between the continental watershed and the line of 

 outcrop of the Fox Hills Gioup, between Separation Station and Raw- 

 lins Springs. These strata have there all their usual lithological charac- 

 teristics, and their aggregate thickness is estimated at not much if any 

 less tlian 4,000 feet. The only fossils found there were a few fragments 

 of Unio and a single imperfect specimen of Viviparus, apparently V. 

 trocliiformis Meek «& Hayden. These were obtained near the railroad 

 about a mile eastward from Separation Station. 



GENERAL DISCUSSIOK 



The desirability of extending an established classification of strata over 

 the whole of a great region or portion of a continent is two manifest to 

 need comment. Within limited areas the lithological characteristics of 

 strata are, as a ride, alone sufficiently constant for the ready recognition 

 of natural groups ; and in the Western Territories there is so unusual a 

 degree of constancy in this respect that certain of the established 

 groups of strata can thus be satisfactorily recognized over very large 

 areas. But even in the most favorable cases of this kind the fossil con- 

 tents of the groups are the most trustworthy guides to their identity ; 

 while for their recognition over large or separate regions, their fossils 

 are almost the only guides worthy of confidence. It is in view of these 

 facts that the present plan for x)aleontological field-work has been estab- 

 Ushed, the present rei)ort being that for the first season's labors of this 

 kind. 



The value of fossil collections for the purposes just indicated depends 

 upon two circumstances, namely, the geographical distribution of the 

 species and types, and their geological or vertical range ; and for the 

 piu'pose of giving a sjnaoptical view of the species collected during this 

 season's labors, together with their geographical distribution, the two 

 following general tables have been prepared, the one of Cretaceous, 

 and the other of Laramie fossils. Similar and equally instructive tables 

 of any and all other groups might be prepared, but the present object 

 is to eml)race only the results of my field observations for the year 1877. 



While there are, as has akeady been shown on preceding pages, 

 some important exceptions to the rule of constancy of paleontological 

 characteristics of both the Laramie and Fox Hills Groups, the residts of 

 this season's labors give grea^t reason to hope that a perfectly harmo- 

 nious classification may be established for the strata of both these and 

 other epochs over the greater ])art of the national domain. These in- 

 vestigations have been quite sufficient to show that the grouping of the 

 Cretaceous strata which was proposed several years ago by Hayden and 



* This species has not becii liitherto described, but may be cbaractevized thus : Shell 

 small, discoid ; volutious six or seven, very slender, coiled closely and almost exactly 

 in a plane so tliat the upper side is known only by the greater backward obliquity of the 

 strife of growth ; their transverse diameter a little greater than the vertical ; surface 

 smooth or marked only by the ordinary striai of growth. Diameter of the coil of the 

 largest example discovered, 8 millimeters ; transverse diameter of the last volution, Ij- 

 millimeters. 



