498 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



Anchitheeium ageeste, Leidy, Eeport U. S. Geol. Surv. Terrs., (ito,) 



I, p. 251, PI. vii. 



A portion of the left mandibular ramus, with the last two true molars 

 remaining, indicates a larger species than any of the preceding, and not 

 materially different from that described by Leidy as above. 



AETIODAGTYLA. 



OKEODON, Leidy. 



Eemaius of species of this genus are exceedingly abundant in the 

 Miocene of Colorado, but represent but two species, which are identical 

 with those already known from Dakota and Nebraska. 



Oeeodon culbeetsonii, Leidy, Ext. Fauna Dak., &c., p. 86. 



Oeeodon GEACiLis, Leidy, Ext. Fauna Dak., &c., p. 94. 

 About one specimen in ten of the genus belongs to this species. 



POEBEOTHBRIUM, Leidy. 



Extinct Fauna Dakota, &c., p. 141. 



This genus differs from AmiyJiitragtilus, Pom., in the association of the 

 first inferior jjremolar with the canine and incisors rather than with the 

 remaining i^remolars. Dental formula I., |- ; C, J- ; P. m., f ; M., § ; the 

 diastema between the first and second premolars only ; canine more or 

 less approximated to the first incisor. The arrangement in the anterior 

 part of the ujjper jaw has not yet been described. There is a diastema 

 behind the first premolar, and one in front of it. The canine is a weak, 

 simple-crowned tooth, and is immediately preceded by a large, canine^ 

 like, exterior incisor. The existence of other superior incisors cannot 

 be demonstrated in my specimens. 



As already known from the descriptions of Leidy, the otic bullse are 

 enormously expanded. Their walls are either occupied with an extensive 

 diploe, or lined with a cancellous layer, which gives a reticulate net- 

 work or section. The osteology of this genus presents a number of 

 interesting features. The cranium only has been described by Professor 

 Leidy. The following observations are based on portions of several 

 skeletons, which include the maxillary, mandibular, and other cranial 

 bones, which I extricated from the matrix myself. The dentition agrees 

 with that figured and described by Leidy. 



The atlas is rather broader than long, with thin diaparapophyses, 

 pierced by the usual foramen at the middle of the base, and .produced 

 well backward at the outer margin. The articular facets of the axis are 

 continuous below the foramen dentale. The neural arch is regularly 

 convex and without keel on its jjosterior .4 ; but the anterior .6 consists 

 of a flat facet descending obliquely to the neural canal, with a median 

 keel and prominent lateral angle descending to the base of the diapo- 

 physis in front. The third and fourth cervical vertebrae are enlarged 

 and quite elongate, and present the usual peculiarity of the Camelid(B in 

 the position of the canal for the vertebral artery. It perforates a part 

 of the base of the neurapophysis, and not that of the diapophysis. , The 

 latter is a decurved lamina, extending the entire length of the centrum, 

 and sending a strong angular ridge from the posterior outer angle to the 

 interior zygapophysis. " The zygapophyses are connected by a strong 



