SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 367 



the caudals, the tail of Prothylacynus must have been not only longer 

 but much thicker at the base than in Thylacynus. It is possible that too 

 much flexibility has been given to it in the drawing of the restored skele- 

 ton. The floor of the neural canal in the cervical, dorsal, lumbar and 

 anterior caudal vertebrae is keeled and perforated on either side of the 

 keel by a large foramen, as in Thylacynus and Borhycena. The epiphyses, 

 although more or less annular, as in Borhycena, do not have such promi- 

 nent bosses from the centra projecting through the median perforations. 

 Frequently the latter are absent. 



Fragments of several posterior ribs are preserved, but are too incom- 

 plete to describe. They have about the same dimensions as the posterior 

 ribs in Thylacynus. The incomplete presternal segment represented in 

 fig. 6, PI. LI, is considerably smaller than that of either Thylacynus or 

 Borhycena, differing from both in possessing a strong, inferior, median 

 keel. 



Appendicular Skeleton. The scapula (PI. XLIX, figs. 2, 2a) is shorter 

 than in Thylacynus, with thicker neck and longer coracoid process. The 

 surface of the supraspinous fossa is convex, in contrast with the approx- 

 imately plane surface of this region in Thylacynus. In part, the con- 

 vexity has been accentuated by crushing. The glenoid cavity is almost 

 circular in outline. The coracoid process projects greatly below the 

 glenoid margin. Its anterior border is strongly inflected. The infra- 

 spinous fossa is somewhat wider than in either Borhycena or Thylacynus. 

 As in those genera, the axillary border is strongly deflected outwardly. 

 The spine has been broken, destroying the acromion. The border of the 

 suprascapular notch is perforated by a small foramen. 



The humerus (PI. XLIX, figs. \-\b] is shorter than in Thylacymis but 

 much heavier. The head is very broad and projects considerably beyond 

 the posterior surface of the shaft. The greater tuberosity rises slightly 

 above the head. The lesser tuberosity is low, not as sharply separated 

 from the head as in Thylacynus. A third tuberosity rising from the inner 

 side of the shaft below the lesser tuberosity probably marks the point of 

 insertion of the coraco-brachialis muscle. The deltoid ridge is long and 

 powerful, extending at least two thirds the length of the shaft. The distal 

 end is exceedingly broad, with greatly enlarged supinator ridge and elon- 

 gated inner epicondyle. The condylar surface is much narrower antero- 

 posteriorly than in Thylacynus, but considerably wider transversely, with 



