Prof. 0. C. Marsh — American Jurassic Mammals. 291 



VIII. Figure 7. A second specimen referred to this species is the 

 anterior part of another jaw, shown in Figure 6. The former jaw 

 shows a weak canine (a), followed by three premolars, each with 

 two fangs. Behind these, in place of the large, trenchant premolar 

 seen in Bryolestes and Stylacodon, is a small tooth, which from its 

 shape may be regarded as the first molar. The remaining teeth 

 agree in their more important characters with the corresponding 

 molars of Bryolestes. The second specimen, Figure 6, shows a 

 similar weak canine, and in front of it, the four incisors in place, 

 increasing rapidly in size forward, the front one being larger than 

 the canine. 



AsTHENODON SEGNis, Marsh. 



In the genus Astlienodon, the inferior dentition on each side is as 

 follows : — Incisors 4 ; canine 1 ; premolars 3 ; molars 8. 



The largest tooth in the entire lower series is the first incisor, 

 Plate VIII. Figure 6, i. The remaining incisors decrease in size 

 backward, as shown in the same Figure, ^, 3, and 4. The canine (a) 

 is small and weak, and its crown resembles that of the incisors. It 

 is implanted by two roots, which are nearly connate. The three 

 premolars behind the canine have each two fangs, and increase in 

 size from first to last, as shown in Figure 7 of the same Plate. The 

 following seven teeth, judging from the shape, are molars, and 

 behind them is the alveole of one more. These molars agree in 

 general form with those of Bryolestes. The form of the lower jaw 

 also is similar in the two genera. The upper jaw of this genus is 

 not known. 



The specimens representing this species indicate an animal about 

 the size of a Weasel. They are from the Atlantosaurus beds of 

 Wyoming Territory. 



Laodon. 



A fourth genus, Laodon, while agreeing in the general type of 

 lower molar teeth with the above forms, differs widely from them in 

 other respects. The molars in this genus have the outer main cone 

 high and pointed, as in the above genera, but the inner opposite 

 cusp is greatly reduced in size, as shown in the type specimen 

 represented in Plate VIII. Figure 5. There appear to have been 

 eight molar teeth, six of which are well preserved. In front of 

 these, are two premolars of nearly equal size, and between these 

 and the canine, there were apparently three more, each with two 

 fangs, making thirteen teeth in the premolar and molar series. The 

 canine had two roots, and the last incisor was placed closely in 

 front of it. 



In this specimen the dental foramen is situated below the summit 

 of the coronoid process. Its aperture is placed obliquely, opening 

 backward and upward, and from its outer margin, the deep 

 mylohyoid groove extends forward and downward, rapidly descend- 

 ing below the lower border of the ramus. 



This lower jaw is intermediate in form between Bryolestes and 

 Stylacodon. It has the slender straight ramus of Stylacodon, with 



