220 



The coracoid process ( ftate XXX., c), arising from a oase of 3 inches in extent, is sub- 

 cnnipressed, with the outer surface concave as it extends toward the end of the process, 

 which, however, is broken off. The upper or front " costa" (ib. g) describes a strong con- 

 cave curve as it recedes from the coracoid; the middle third of its extent (ib. g'), which 

 was probably convex and produced, has been broken away. Where it is again entire 

 (ib. <f) it describes a gentle concavity, and forms the outer border of a sudden thickening 

 of that part of the basal end of the scapula. The upper or basal three-fourths of this 

 anterior border of the bladebone are curved " dorsad," so as to bound or form the (trans- 

 verse) hollow of the supraspinal fossa (ib. fig. 1, i). The part broken from the upper 

 costa (g 1 ) may have made the breadth of the fossa, as in the Kangaroo and most Mar- 

 supials, greater at its mid part than appears in this fossil. 



The subspinal fossa (ib.j) increases in breadth from the basal (b) to the articular end 

 (a) of the scapula, singularly reversing its proportions in other Mammals. In the Koala 

 (PhaHColarctos) this fossa retains its breadth through an extension of the lower costa 

 nearer to the glenoid cavity than usual*. The corresponding extension is proportion- 

 ally greater in the scapula of the Megatherium ; but in Diprotodon it is continued as 

 far forward as the neck of the scapula, with an increase of thickness, and a bend toward 

 the "dorsum" of the scapula, increasing the depth of the concavity of this part of the 

 subspinal fossa. The border of the plate (ib. a, a!) produced below or behind the gle- 

 noid cavity (d), and having the same aspect, is very thick, concave lengthwise, convex 

 across, with a rough slightly projecting insertional surface at its middle : a more rugged 

 surface appears also at the angle a, where it joins the lower or hinder costa, but this is 

 the seat of some mutilation of the fossil. 



This costa (a, b) loses thickness as it recedes from the angle for one-fourth of its extent ; 

 it regains a certain thickness and ruggedness for another fourth (k), where it is also bent 

 toward the subscapular plane ; it then continues drawing nearer to the origin of the spine 

 and finally thickens as it is lost in the obtuse contracted basal end (b) of the bladebone. 



The long and narrow subscapular surface (Plate XXX. fig. 1 I) presents a gentle 

 concavity lengthwise, with a corresponding convexity across the middle, rather increased 

 at the two ends ; the mid convexity is changed to a concavity by the in-bending of the 

 part (k) of the lower costa above mentioned, The smooth subscapular surface is broken 

 only by the thick short triangular elevation (ib. m) extending from near the upper or 

 fore angle of the base. 



The singular, not to say unique, development of the "glenoidal" part of the inferior 

 costa {a, a') or subspinal plate, was doubtless correlated with some peculiarity of use or 

 application of the fore limb. As to the general shape of the scapula, I cannot suppress 

 expressing the interest with which I have viewed in this old extinct Marsupial or im- 

 placental form of Mammal the retention of so much of the archetypal or pleurapophy- 

 sial proportions which one sees without surprise in inferior Vertebrates such as Mono 

 tremes, Birds, Reptiles, and Fishes. 



* This peculiarity is figured in the ' Cyclopaedia of Anatomy,' Art. Marsvpialia, p. 281, fig. 106. 



