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



The Loiver Teeth. 



Among the many lower incisors of this group found in this 

 horizon, three well-marked types, at least, may be distinguished, 

 as follows : 



The striate forms, which have the inner face flattened to 

 meet its fellow, and the entire crown covered with enamel. 

 This type is figured on Plate III of Part I, and Plate VIII of 

 Part II. Another is shown on Plate VI, figure 5, of the present 

 paper, and there are several others. 



The second series is somewhat smaller in size, with the crown 

 also covered with enamel, but it is compressed, and irregularly 

 bevelled on the concave side, somewhat like the lower incisors 

 of Hypsiprymnus. One of these teeth is represented on 

 Plate IV, Part I, and another is shown below, Plate V, 

 figure 4. The enamel is very thin, and easily lost. 



A third type, smaller than the above, is more abundant. It 

 is slender, considerably curved, and the outer face only is 

 entirely covered with enamel. This type is seen on Plate VIII, 

 Part II, and in figure 4, Plate VI, below. 



A fourth type, which may pertain to the upper series, is 

 similar to the second, but has one margin serrate. This 

 form is shown on Plate VII, Part II. A modification of it 

 occurs in smaller forms, one of which is shown in Plate 

 VII, figure 2, below. The lower incisors of some of the exist- 

 ing shrews (Sorex) are notched on the outer margin in a 

 somewhat similar manner. 



In all the Allotheria of this horizon, the first and second 

 lower premolars are apparently wanting. Only the third and 

 fourth have been found in place. The third when present is 

 very small, and cylindrical, with a single root. The fourth is 

 very large, with two unequal roots, and with a compressed, 

 cutting crown. As a rule, this crown is very low, its summit 

 being elevated but little above the true molars behind it. 

 This is an unexpected feature, as the Jurassic forms below and 

 those in the Tertiary above both have the cutting fourth 

 premolar elevated high above the molar series. 



In • the known Cretaceous forms, the crown of this tooth 

 shows wide variations, indicating several distinct genera and 

 species. In one series, the summit is deeply notched, and 

 strong ridges cover the greater part of the outer and inner 

 faces. In another, the superior notches are smaller or wanting, 

 and the surfaces of the two sides are marked by a larger 

 number of ridges and grooves. In some of these forms, the 

 crown is very low, not more than half as high as long. 



