Notices of Memoirs — A New Jurassic Mammal. 371 



the relations of these two rocks, noting especially whether, as in the 

 above instances, the gabbro is the newer rock. 



Another point of interest in connexion with these serpentines and 

 gabbros is that they are, beyond doubt, of late Cretaceous or early 

 Tertiary age, 1 and yet are practically identical with serpentines and 

 gabbros which are almost as certainly of Palaeozoic age. 



1TOTIGES OIF 1 HVCDEZMZOHEiS. 



I. — A New Jurassic Mammal. By Professor 0. C. Marsh. 

 (From the American Journal of Science and Arts, vol. xviii. July, 1879.) 



^URING a recent visit to the Rocky Mountains the writer spent 

 some time in examining the deposits known as the Atlantosaurus 

 beds, and was rewarded by the discovery of several interesting fossils, 

 among them the lower jaw of a small mammal. This specimen indi- 

 cates a diminutive marsupial, quite distinct from the one previously 

 described by the writer from the same horizon (Dryolestes prisons), 2 

 which has hitherto been the only mammal known from the Jurassic of 

 this country. 



The present specimen, which is from the left side, has the larger 

 part of the ramus preserved, with a number of perfect teeth in position. 

 Most of the symphysial portion is lost, and the posterior part is 

 missing, or only faintly indicated. The jaw was remarkably long and 

 slender. The horizontal portion is of nearly equal depth throughout, 

 and the lower margin nearly straight. The form of the coronoid 

 process, condyle, and angle of the jaw cannot be determined from this 

 specimen. The remarkable feature in this jaw is the series of premolar 

 and molar teeth. These were very numerous, apparently as many as 

 twelve in all, and possibly more. The premolars had their crowns 

 more or less compressed, and recurved, and some of them were 

 supported by two fangs. These had a small posterior tubercle at 

 the base of the crown, but none in front. The molar teeth were all 

 single-fanged, with elevated conical crowns. Those preserved have a 

 distinct cingulum. The molars increase in size from the first to the 

 fifth. All the teeth preserved have the crowns raised considerably 

 above the upper margin of the jaw, and thus appear to be loosely 

 inserted. A large pointed tooth lying near the jaw appears to be a 

 canine. The principal dimensions of this specimen are as follows : 



Length of portion of jaw preserved 115 mm, 



Extent of five molar teeth 4 - 



Extent of entire molar series 5* 



Height of fifth true molar above jaw 2" 



Depth of jaw below fifth molar 1*75 



Depth of jaw below last premolar l - 5 



Depth of jaw below first premolar 1-4 



In comparing this interesting fossil with the forms already known, 

 it is at once evident that it differs widely from any living type. Its 



1 Jervis, Quart. Journ. Geol. Soc. vol. xvi. p. 480, states that the "diallagic ser- 

 pentine " pierces the Upper Cretaceous but not the Tertiary rocks. Stoppani, Corso 

 di Geologia, vol. iii. § 704, places it, if I rightly understand, at about the same 

 period. Both speak of a compact serpentine "without diallage," of Miocene age. 

 This I have not seen. See also D'Achiardi, vol. ii. p. 181. 



3 Silliman's Journal, vol. xv. p. 459, June, 1878. 



