42Q PEOFESSOE OWEN ON MACKOPUS. 



also a small intervening bit of the squamosal (PI. LXXIV. fig. 4, 27) ; inferiorly it 

 unites by its longest borders with the exoccipitals (ib. 2,2') by straight sutures, slightly 

 descending as they converge to the foramen magnum (ib. 0), to which the shortest and 

 lowest border originally contributes. 



The exoccipitals and their paroccipital productions unite laterally with the mastoids 

 (ib. 8), below which the paroccipitals receive the alisphenoid abutments (ib. fig. 1, 6'). 

 The basioccipital unites by its postlateral borders with the exoccipitals, by its prelateral 

 borders with the petrosal and alisphenoids, with more or less interruption through 

 the intervening postlacerate fissures (ib. fig. 3, q), and by its fore border with the basi- 

 sphenoid. The contiguous or conjoined borders of these elements usually swell out so 

 as to produce same convexity at this part of the base of the skull. 



The basisphenoid (ib. fig. 3, 5) loses breadth as it extends from the basioccipital 

 forward ; its sides are pierced or deeply grooved obliquely by the entocarotids which 

 have impressed the contiguous part of the basioccipital ; the entocarotid canal, com- 

 mencing at the fore part of the postlacerate fissure, is completed by the base of the 

 alisphenoid. The pair of canals converge upward and forward, and terminate within the 

 cranium at the hind part of the " sella," separated there by a bony tract not exceeding 

 the diameter of the arterial canal. 



The basisphenoid is longer than the basioccipital ; it unites with the third cranial 

 centrum (9) beneath the vacuity common to the prelacerate and optic foramina. 



The alisphenoid ( 6 ) forms a large outward swelling for the lodgment of the natiform 

 protuberance of the cerebrum, the pressure of which reduces the neurapophysial plate 

 to almost transparent thinness. Below the convexity as large a concavity is formed, 

 with an irregular surface, for the attachment of the pterygoid muscles, the cavity being 

 completed by the proper pterygoid bone (ib. fig. 3, 24). The base of the alisphenoid 

 a)'ticulates with the side of the basisphenoid, leaving a small hind part of that element 

 clear for the entry of the entocarotid, and as small a fore part for the exit of the optic 

 nerve. From tliis attachment it extends backward and forward, attaining an antero- 

 posterior extent equalling that of the parietal. The alisphenoid is pierced by the 

 " foramen ovale " {tr) opposite the fore end of the petrosal, and by the " foramen 

 rotundum " {tn) near the fore part of its attachment to the basisphenoid. The ali- 

 sphenoid gains thickness and loses breadth as it extends backwards outside the cranial 

 walls to articulate with the squamosal (27), tympanic (28}, mastoid ( 8 ), and paroccipitals 

 (4), and contributes to form the tympanic cavity'. The thinner cranial plate ascends 

 anterior to the squamosal to join the parietal, frontal, and orbito-sphenoid. 



The sagittal suture (ib. fig. 2, 7') persists until maturity, its obliteration beginning 

 behind. Here the parietals (7) join the interparietal ( 3 a-) and superoccipital (s'), 

 an intervening " fontanelle " long remaining, reduced to a fissure which is continued 

 between the mastoid and parietal ; it is usually obliterated in aged Kangaroos. 



' Art. " Marsupialia," op. cit. torn. iii. p. 271. 



