350 PEOFESSOE OWEN ON THE SPECIES OF PHASCOLOMTS'. 



of a bare-nosed Wombat, as to whether it belonged to Phascolomys platyrhinns or Phas- 

 colomys vomhatus. 



The basal view of the skull oi Phascolomys jplatyrhinus (PI. LII. fig. 1) shows one of 

 the degrees of variation to which the basioccipital is subject in the depth and breadth 

 of the emargination of the part contributing to the lower border of the foramen magnum. 

 In this skull the emargination is narrow and deep ; in a second skull of like size it is 

 less deep and wider. The basioccipital is subject to the same variety in Phascolomys 

 vomhatus. 



In two skulls of Phascolomys latifrons the emargination of the lower border of the 

 foramen magnum is wider, as in the subject of fig. 1, PI. LIII., than in the two first- 

 named species. In all the three species, the under surface of the basioccipital has a 

 median longitudinal ridge, which slightly varies in its depth and sharpness ; there is a 

 shallow vacuity on each side of the ridge. Each exoccipital, where it coalesces with 

 the basioccipital, develops a tubercle, which, in the platyrhiae (PL LII. fig. 1, b) and 

 Tasmanian Wombats, abuts against the petrosal. In Phascolomys latifrons (PI. LIII. 

 fig. 1, b) the corresponding (exoccipital) tubercles are more prominent and project freely 

 below the petrosals (le), resembling the pterapophyses of the basisphenoid in bii'ds. 

 The exoccipital is perforated anterior to the condyle by, commonly, two hypoglossal 

 foramina; these are more equal in size in Phascolomys latifrons than in the other 

 two species. There is usually a small vascular foramen external to the upper end of 

 the condyle. The wedge-shaped petrosal (ib. fig. 1, le) abuts against the side of the 

 basioccipital, with the thin end directed forward. The squamosal (ib. 27) expands at the 

 inner side of the mandibular articular surface (g) to form a tympanic cell or ' bulla,' 

 which is large and widely open backward, receiving the inner orifice of the tympanic 

 (ib. 28) in Phascolomys j)latyrhinus and P. vomhatus. In Phascolomys latifrons this 

 pre- or antetympanic cell of the squamosal (PL LIII. fig. 1, 27) is smaller than in the 

 Tasmanian Wombat, much smaller than in Phascolomys ])latyrhinus. External to this 

 cell the squamosal develops, in the bare-nosed Wombats, a vertical ridge, which is 

 wedged into a groove of the tympanic ; it is scarcely marked in the hau'y-nosed Wombat. 



In Phascolomys latifrons (PL LIII. fig. 4) the articular bar of the squamosal {g) is 

 relatively shorter than in the platyi-hine or Tasmanian Wombat ; and its inner end is 

 notched posteriorly, which receives and is reciprocally received by a notch in the fore 

 and outer part of the tympanic (28). This bone sends forward a thick triangular 

 plate, contracting to the part which is notched for the squamosal, in a way which 

 off'ers a close and interesting analogy to the ' gomphosis ' of 28 with 27 in Birds. 



The more marked division of the supratympanic cell in Phascolomys latifrons (ib. 

 fig. 4, I, m), and the greater size and depth of the anterior portion or cavity (ib. I), are 

 noted in the cranial characters of the species, and have been found in all the skulls 

 examined since the date of my second Memoir '. 



' Traus. Zool. Soc. vol. iii. (1845), p. 303. Tlie " enormously excavated supratympanic cells " oiPh. latifrons, 

 are noticed by Dr. Murie, loc. cit. (1865), p. 842. 



