306 



well-defined lacrymal tubercle (ib. 7.$) defining the fore and upper part of the orbit, 

 are common to both the bare-nosed species, and distinguish them from Pliascolomys 

 latifrons. 



The characters of the nasal bones (PI. XLV1I. figs. 1, 3, 4, 5, 15) in regard to shape, 

 size, and connexions are detailed in a succeeding section (pp. 315-319). 



The basal view of the skull of Pliascolomys platyrhinus (PL XLVIII. 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 Pliascolomys vombatus. 



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

 foramen magnum is wider, as is shown in the subject of fig. 1, PI. XLIX., 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 platyrhine (PI. XLVIII. fig. 1, b) and 

 Tasmanian Wombats, abuts against the petrosal. In Pliascolomys latifrons (PL XLIX. 

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

 below the petrosals (ie), resembling the pterapophyses of the basisphenoid in birds. 

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

 foramina ; these are more equal in size in Pliascolomys 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, 16) 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 Pliascolomys platyrhinus and P. vombatus. In Pliascolomys latifrons this 

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

 Tasmanian Wombat, much smaller than in Pliascolomys plat yrldnus. 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 hairy-nosed Wombat. 



In Pliascolomys latifrons (PI. XLIX. fig. 4) the articular bar of the squamosal (g) is 

 relatively shorter than in the platyrhine 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 

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



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

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

 among the cranial characters of the species. 



