THE 



GEOLOGICAL MAGAZINE, 



NEW SERIES. DECADE III. VOL. IV. 



No. VII.— JULY, 1887. 



OI^Ia-I^s^.A.XJ .A.iaTioXiE3S. 



I. — American Jurassic Mammals. 



By Professor 0. C. Marsh, M.A., LL.D., F.G.S. 



(PLATES VIII. AND IX.) 



{Concluded from page 247.) 



Drtolestid^. 



THE first American mammal found in the Jurassic, and a large 

 proportion of those since discovered, belong to a peculiar family 

 which the writer has called the Dryolestidcs. It includes several 

 genera and numerous species from America, and is likewise repre- 

 sented among the forms found in Europe. 



The type species of the group, Dri/olestes prisons, was based upon 

 a characteristic lower jaw, although the 8j)ecimen was imperfect. 

 A nearly complete lower jaw of the species is represented on Plate 

 VIII. Fig. 2. An allied species, Dryolestes vorax, is shown on the 

 same Plate, by Figures 3 and 4. Stylacodon, Asthenodon, and Laodon, 

 other genera of this family, are likewise represented on the same 

 Plate. 



The upper jaws of several genera of this family are now known 

 with tolerable certainty, and these will be figured and described 

 fully in the Memoir now in preparation. 



All the genera of this family have more than the typical number 

 of teeth (44), and the general characters of the inferior dentition 

 are well shown in Plate VIII. The lower teeth form a close-set 

 series, without diastemas, or marked interspaces. 



There are three, or four, incisors, and in those preserved, each 

 has a distinct crown, and the series diminishes in size from in front 

 backward. The canine is inserted by two fangs, more or less 

 distinct, and in most forms its crown is prominent and trenchant. 

 Three or four premolars follow, increasing in size backward, with 

 the last usually very prominent, and in some forms larger than the 

 canine. These premolars all have two roots, and a compressed 

 crown. All have one main cusp, and a small posterior heel. There 

 is usually a small anterior cusp, especially on the posterior teeth. 



The molar teeth are from six to eight in number, and are essen- 

 tially identical in form, and usually distinct from the premolars. 

 The crown consists of one main external cone, high and pointed, 

 and three internal cusps, which vary much in development in the 

 difierent genera. Seen from the outside, these teeth appear to be 



DECADB III. VOL. IV. NO. VII. 19 



