24 



POEBROTHERIU3I VILSONII Leidj, 1. C, 111. 



Several specimens obtained. This genus differs from Ampliitracjulus 

 Pom., in the association of the first premohir, with the canine and incis- 

 ors, rather than \Yith the remaining premolars. Dental formula : I. 3, 

 C 1; P. M. 4; M.S. The diastema between the first and second pre- 

 molars only ; canine more or less approximated to the first incisor. 



The osteology of this genus presents a number of interesting features. 

 The cranium only has been described by Professor Leidy. The follow- 

 ing observations are based on portions of a skeleton, which include the 

 maxillary, and part of the mandibular, and other cranial bones, which I 

 extricated from the matrix myself. The dentition agrees with that fig- 

 ured 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 den tati. The neural arch is regularly 

 convex, and without keel on its posterior A, but the anterior .(3 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 diai)oph- 

 ysis in front. The third and fourth cervical vertebrae are enlarged, and 

 quite elongate, and present the usual peculiarity of the CameUdae 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 

 sends a stroug angular ridge from the posterior outer angle to the ante- 

 rior zygapophj'sis. The zygapophyses are connected by a strong longi- 

 tudinal angular ridge. The neural spine is a prominent keel of no great 

 elevation. The hypapophysis is an acute keel, low in front, but pro- 

 duced downward and backward to a rugose obtuse extremity. The 

 centra are slightly opisthocoelian, the articular surfaces so moderately in- 

 terlocked as to constitute a form intermediate between that of the camels 

 and of the Macrauchenia. An anterior dorsal vertebra is more strongly 

 opisthocoelian, resembling that of the lama. The diapophysis has a reni- 

 form tubercular surface, which looks downward; from its posterior in- 

 ferior angle a strong fold-like ridge originates, and is continued as the 

 posterior margin of the neural arch. Below the capitular facet a short 

 ridge originates, which incloses a median fossa with its fellow on the an- 

 terior half of the centrum. A lumbar exhibits a strongly depressed cen- 

 trum and the absence of an epiphysis from it, and from the dorsal de- 

 scribed, indicates the immaturity of the individual. 



The humerus is little expanded distally, and is truncate from the 

 trochlear margin on the inner side. The posterior portion of this face 

 is produced into a strong tuberosity, of which a trace may be observed 

 in the lama, which pievents the extension of the forearm beyond an angle 

 of 180°. The inner trochlear face has the greater sweep and less width, 

 and is uninterrupted ; the outer is wider, and is divided into two nearly 

 coincident planes. There is a supracondylar foramen. 



The forearm is long and slender, and the ulna coossifled its entire 

 length, except a foramen near its distal end. The medullary cavities of 

 the two bones are separated for the proximal half of their length. 

 A shallow groove distinguishes the ulna proximally, and at the middle 

 of the shaft the latter forms an acute edge. Distally the combined 

 bones present three planes, two lateral and a median. The lunar facet 

 is most impressed ; the scaphoid and unciform are equally prominent. 



The carpus consists of eight bones, the entire mammalian number, all 

 entirely distinct. The second series presents the most important peculiar- 



