375 



however, by subsequent growth contract the fissure, and in adults of some large species 

 obliterate it and complete the foramen by the exclusion of the superoccipital. There 

 are two or more precondyloid foramina (ib. fig. 3, p), anterior to which is the vagal 

 foramen (v), and next the larger single jugular notch, through which may be seen 

 the hind end of the petrosal (i6) ; this notch is usually completed, as a foramen, by the 

 paroccipital process of the alisphenoid 6', anterior to which the fore part of the petrosal 

 is visible ; but in some the union with the exocctpital (ib. fig. 2) does not take place, 

 and the whole extent of the basal part of the petrosal is seen ; the junction of the ali- 

 sphenoid (ib. fig. 1,6') with the paroccipital (4) is constant and extensive. A smooth 

 channel curves round the upper part of the condyle (ib. fig. 4, g) between it and the 

 base of the paroccipital (4). The superoccipital (ib. fig. 4, 3) is octagonal, the upper 

 and lower borders being the shortest ; the lateral ones next these above and below are 

 the longest; the two outer sides are of intermediate extent. There are two rough 

 oblong depressions (ib. 3,*) near the upper lateral borders, for tendinous attachments. 

 Superiorly the superoccipital joins the interparietal (ib. fig. 2. 3), varying in shape and 

 composition; laterally it joins the parietal (7) and mastoid (8 ), and in some species 

 also a small intervening bit of the squamosal (PI. LXVI. 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 produc- 

 tions 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 basisphenoid. 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 pre- 

 lacerate 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). 



o — * 

 O / * 



