PROFESSOE OWEN ON THE EOSSIL MAMMALS OF AUSTRALIA. 



543 



As in this region the vertebrae in many Mammals decrease in breadth before regaining 

 the size which then goes on augmenting to the lumbar region, I first take for description 

 that (Plate XLIV. figs. 5 & 6) which with a broader centrum has a shorter as well as 

 broader spine. The fore-and-aft extent of the centrum is 2 inches at its lower part ; it 

 slightly decreases towards its upper surface. The breadth of the centrum is 4 inches 10 

 lines, above which this dimension is increased by the share contributed by the neurapo- 

 physes (ib. fig. 5, n, n) to the body of the vertebra (6') ; the sutural lines indicative of this 

 share are plainly traceable on the terminal articular surface (ib. fig. 5, c), from which the 

 epiphysial plate has become detached. As the ends of both diapophyses and neural spines 

 are broken off, the following dimensions of the vertebra are not the full ones, viz. of 

 breadth 8 inches 6 lines, of height 9 inches 6 lines. The width of the neural canal is 

 nearly 4 inches, its height is fully 3 inches. Both articular surfaces of the centrum are 

 nearly flat, the anterior one in a very slight degree convex ; but both surfaces are epi- 

 physial, with coarse furrows and lines affecting a radiate disposition, the extent of which 

 rugosity indicates the complementary plate to have overlapped both elements of the ver- 

 tebral body, viz. the neurapophysial (fig. 5, w, n) and the central one (ib. c)*. 



A prominence on the upper third of the side of the body indicates the lower boundary 

 of the neurapophysis, and this part of the body holds the main part of the impressions 

 [pi, pi') for the head of the rib, of which impressions the hinder is the larger. The 

 contour of the articular surfaces of the body is semicircular. The neurapophyses, after 

 contributing their share to the vertebral body, extend upward, outward, and a little 

 forward, contracting into subcylindrical pedicles which suddenly expand to send off 

 the diapophyses (ib. figs. 5 & 6, d), prezygapophyses (z), and postzygapophyses 

 Before developing the latter processes the neurapophyses begin to bend inward, still 

 ascending ; then they contract, especially from before backward, converge, and coalesce 

 to form the base of the antero-posteriorly compressed and laterally expanded spine [ns). 

 The base of this spine is strengthened both before and behind by a low broad median 

 ridge ; its terminal divisions diverge as they rise. The undivided base forms a low 

 obtuse eminence between them (fig. 5, ns). 



The prezygapophysis (fig. 6, z) projects forward as a semicircular shelf, the flat arti- 

 cular surface looking upward, with a very slight inclination downward and outward ; the 

 postzygapophyses [z'), of somewhat smaller size, are supported each by a buttress of bone 

 descending from the hinder and outer angle of the spine, and expanding with a promi- 

 nent convexity to the articular surface. The upper surface of the centrum, between the 

 neurapophysial bases, shows the large venous fossa. 



In the other dorsal vertebra (Plate XLIV. figs. 7, 8) the diapophysis is entire on the 

 left side (fig. 8, d), and expands into a protuberance with an articular surface (fig. 7, d) 

 1 inch 9 lines by 1 inch 3 lines, for the tubercle of the rib on its lower half. The 

 neurapophysial parts of the centrum are traceable, and make a more definite rising at 



* This is tlie usual character of epiphyses completing compound bones, as, e.g., at the proximal end of the 

 three confluent metatarsals in the bii-d, at both ends of the two confluent metacarpals in the ruminant, &c. 

 MDCCCLXX. 4 E 



