0. G. Marsh — Discovery of Cretaceous Mammalia. 253 



In a second series, the whole premolar is smooth, the sides 

 being without distinct grooves or ridges. This type of 

 premolar is found in several forms, which differ much in other 

 features. 



The lower molars are as usual two in number, and although 

 differing widely in the form and structure of their crowns, 

 have only two parallel series of crescents or tubercles, an outer 

 and inner row, with a groove or valley between them. 



The Upper Teeth. 



The upper dental series of the Allotheria of this horizon is 

 much more complicated than the lower series, and as the teeth 

 known are nearly all isolated, or with only portions of jaws 

 attached, their relation to the inferior teeth is at present hardly 

 more than a matter of probability. Some of the larger forms 

 certainly have two, if not three, upper incisors, and the main 

 one is deeply notched at one or more points, as in the genus 

 Allodon of the American Jurassic. 



The upper premolars that can be placed with certainty are 

 in most cases compressed in form, and evidently met the 

 cutting fourth premolar below. Most of these upper cutting- 

 teeth secured, however, pertain to the smaller forms, making 

 it very probable, at least, that in the large species, the upper 

 premolars were not much compressed, and had in themselves 

 no cutting function. This was certainly true, also, in some 

 of the smallest forms; as in Allacodon pumilus, the premolars 

 are all known, and in one of the specimens figured, three of 

 them are in place (Plate VII, figure 3). 



The upper molars of the group now known from this horizon 

 are of special interest. Although not found in position in any 

 one specimen, so many have been secured with portions of the 

 jaw attached, that their place in the dental series has been 

 ascertained in several forms. All these have apparently but 

 two true molars, the penultimate being in some cases of 

 smaller size than the last. 



The upper molar teeth may be separated into two series, the 

 first having three longitudinal rows of elevations on the crown, 

 and the second series but two rows. The various modifica- 

 tions of these types are numerous, and certainly indicate many 

 distinct forms. In some of these, the prominences in each of 

 the three rows are conical, and each row is complete, especially in 

 the last molar. Another type has the three rows complete also, 



