



ty Pt: , , aa 
. 2 tenn 
' r un . 
eho eae we 
by the discovery of these dinosaurian bones ‘sibs Pea it is “cor 

200 B. N. Zz BOUNDARY “comnsstoy. 


that of the recurrence of plants and fresh-water molluscs of types 
lar to those of the earlier Cretaceous, depended no doubt, on the - 
terrupted continuance of land areas with similar climatic conditions, 
changes which operated in the comparatively rapid exclusion of sucl 
Cretaceous salt-water molluscs as still lived, from the area in questi 
must have tended to spread and perpetuate the terrestrial fauna and sagt 
and the fact of the extinction of the few remaining dinosaurians, while — 
the conditions continued at least as favourable to their existence as in the i ; 
preceding period, shows that they belonged to a type, gradually succumb- _ 
ing to that decay, which seems to effect not only the individual, but the | 
species and the race. This result may perhaps have been accelerated, as 
suggested by Prof. Cope, by an irruption of mammals from some other 
quarter. | ; 
460. The Cretaceous formation cannot be defined as exactly co-exten-— 
sive with any one form of life, however great its classificatory value may 
be supposed to be ; and no classification, of any natural objects or periods, 
4 k, based on a single structure or law, has proved sufficiently facile to include 
| ~ the facts, when all were known. By the comparison of certain conditions 
now existing, and their accompanying forms of life, with those of the _ 












* - 








Cretaceous, we may in a similar way prove its continuance to the present 
day. The vertebrate fauna, when taken as a whole, is not by any means 
an exclusively Cretaceous one, but includes according to Prof. Cope’s _ Ss 
comparative lists,* both in Colorado and Dakota, some forms elsewhere ¢ 
supposed to be characteristically Tertiary. Of the vertebrate remains : 
found in connection with the lowest beds of this series on the forty-ninth 
parallel, Prof. Cope writes :—“This is a characteristic collection of the 
aa reptiles of the Ft. Union Cretaceous, but with increased admixture of 
yy Rocene forms. Plastomenus is an Kocene genus, but the reference of the _ 
’ new species to it is not final. But you send two Hocene gar scales which 
‘ have every appearance of belonging to the same formation. Will you — 
re-examine your notes to inform me whether they really belong to the — 
same horizon as the others?’} The gar scales referred by Prof. Cope to 
the genus Clastes, were obtained at the very base of the Lignite forma- 
















‘ia * U.S. Geol. Surv. Territ., 1873, p. 433. ¢ 
A t Prof. Cope refers the Green River and Bridger beds, overlying the Lignite formation, to the Eocene. an, 
ary Prof. Cope also says of this collection, in a paper read before the Academy of Natural Sciences, Philadel- Ca 
AG phia :— The list of specics, short as it is, indicates the future discovery of a complete transition from . 
aa Cretaceous to Eocene life more clearly than any yet obtained in the West,” 

