0. C. Marsh — Discovery of Cretaceous Mammalia. 251 



in future to search for the great series of intervening strata 

 containing the forms that lead from one to the other. 



Whether the missing deposits are Cretaceous, transitional, 

 or Tertiary, or all three, they must exist somewhere, for their 

 existence is demonstrated by what is known of vertebrate life 

 below and above them. This knowledge has been mainly 

 secured within the last few years, and has hardly become 

 accessible yet to other departments of science. 



Bearing in mind all that is known to-day of the develop- 

 ment and succession of vertebrate life in America, from the 

 early Silurian on to the present time, it is safe to say that the 

 faunal break as now known between the Laramie and the lower 

 Wahsatch is far more profound than would be the case if the 

 entire Jurassic and the Cretaceous below the Laramie were 

 wanting. Consequently, every discovery that throws light on 

 this dark region of the geological section has an importance 

 beyond its intrinsic value. In mammalian life, the record here 

 is especially imperfect, and every addition should be welcome. 



The Allotheria. 



The most important mammalian remains so far discovered 

 in the Ceratops beds of the Laramie belong to the group 

 which the writer has called the Allotheria* Members of this 

 group are known also from both the Triassic and Jurassic, and 

 their presence in the Cretaceous was, of course, to be expected. 

 Several hundred specimens have now been found, indicating 

 quite a number of genera and species, but the correlation of 

 these remains is as yet a matter of great difficulty. The indi- 

 vidual teeth are most of them in excellent preservation, and, 

 in a few cases, several are in position in the jaw, but accurate 

 determination of many of the forms must depend upon the 

 results of future discovery. The isolated specimens now known, 

 taken together, or the corresponding parts when compared 

 with each other, all prove the abundance and variety of this 

 group during Cretaceous time ; yet it seems much more 

 profitable at present to indicate the main variations in these 

 specimens rather than attempt reconstructions of animals to 

 which they may be imagined to belong. 



As is usually the case with fragmentary remains of small 

 mammals, the lower jaws are best preserved, and readily 

 recognized. The lower teeth seem less rare in the Laramie 

 than any other portions of the skull or skeleton, and as they 

 are especially characteristic in the Allotheria, must be used 

 largely in the present investigation. 



*This Journal, III, vol. xx, p. 239, 1880. 



