522 PEOEESSOE OWEN ON THE FOSSIL MAMMALS OF AUSTEALIA. 



The basioccipital, as it advances, expands and gains a thickness of nearly an inch of 

 compact and minutely cellular bony tissue. 



The occipital condyles (Plate XXXV. fig. 3, 2), an inch apart below and 2| inches apart 

 above, have their lower extremity obtuse, about an inch broad, curved inward and forward ; 

 they expand as they ascend, diverging to a breadth of 2 inches at their upper ends. The 

 vertical convexity of each condyle describes a semicircle, the extent of the articular 

 surface following this curve being 6 inches. The outer border is longer than the inner 

 one, so that the upper margin of the condyle rises obliquely from within outward. The 

 narrow lower ends of the condyles rise or project more abruptly from the intervening 

 basioccipital border of the foramen magnum than in Macropus. Each condyle is here 

 impressed by a rough surface or shallow notch at its inner surface, indicative of tendinal 

 insertion. A low narrow ridge extends from the outer part of the lower end of the 

 condyle, forward, and may indicate the lateral extent of the basioccipital at this part. 

 The transverse convexity of the condyle is greater, less angular, especially at the lower 

 half, than in Macropus. The oblique base or upper end of the condyle projects pro- 

 portionally more from the non-articular part of the exoccipital. A similar better defi- 

 nition or greater prominence characterizes the outer margin of the condyle ; the inner 

 margin forming the sides of the foramen magnum is sharper and better defined than in 

 Macropus : these borders are also less divergent as they rise. The inner, non-articular 

 side of the condyle is slightly concave, rough, subtuberculate. The outer border of the 

 articular part is sharp, and projects over the inner non-articular side of the condyle. In 

 their posterior terminal position and degree of prominence the occipital condyles of 

 Jbiprotodon resemble those in Binotherium. 



The foramen magnum (ib. fig. 3, 0) is bounded above by the arched obtuse border of 

 the exoccipitals, which bones rapidly gain thickness as they extend from the foramen. 

 I assume that the exoccipitals met above the foramen, as in many Marsupials ; or, if not 

 meeting, had their interspace filled by the superoccipital, as in Phascolarctos : they left 

 here no notch, such as one sees in Macropus Bennettii. The upper border of the 

 foramen magnum is non-emarginate in Macropus major ; but it is relatively of greater 

 extent in that Kangaroo, through the greater divergence of the condyles, and it is less 

 arched or concave transversely than in Biprotodon. Two precondyloid foramina open 

 upon each exoccipital, opposite the junction of the lower and middle thirds of the 

 condyle, from which the hindmost foramen is distant 9 lines, the next 1 inch 2 lines ; 

 each foramen is about 4 lines in diameter. An irregular or tubercular ridge curves over 

 the fossa of the precondyloid foramina, expanding to be lost on the paroccipital. This 

 process (ib. figs. 1 & 3, «) is tuberous, thick, and short ; it is not so much produced as 

 in Macropus. 



The foramen magnum is more evase than in Macropus, i. e. it expands funnel-wise to 

 its outlet, backward, and especially above and below ; it has more the character of a 

 short (neural) canal than a foramen, through the antero-posterior extent of its wall or 

 rim. It is transvers(>ly elliptical, 2 inches 6 lines in long, 1 inch 3 lines in short, dia- 



