8 PEOF. T. JEFFERT PARKER ON THE 



The occipital condyle (o.c\ o.c^) is divided into three facets, one large and medio- 

 ventral in position {o.c ^), and two smaller, paired and supralateral {o.c ') ; these 

 correspond with the similar facets on the atlas vertebra (fig. 21, PI. VI., p. 24). The 

 medio-ventral facet is not vertical, but is inclined at an angle of about 45° (figs. 7 and 11). 

 Immediately external to each dorso-lateral facet is a small aperture {x) which pro- 

 bably transmits the glossopharyngeal and vagus nerves. 



The foramen magnum (figs. 10 and 12, f.m) is a small aperture having the form 

 of a long oval, and is considerably inclined forwards. The whole posterior surface of 

 the skull also slopes forwards, and passes almost insensibly into the dorsal surface, 

 which is arched from before backwards, and from side to side. The anterior half of the 

 middle region of the dorsal surface is quite unossified, consisting of the cartilaginous 

 tegmen cranii (figs. 8, 11, and 12, t.cr). This is bounded externally by large paired pro- 

 cesses of the frontals {fr) ; its right and left moieties slope inwards, uniting in a slight 

 longitudinal groove. 



Laterally, the roof of the cranium is bounded by well-marked svpraorhital ridges 

 (fig. 8, fr), from which are continued inwards large supraorhital plates (figs. 7 and 9, 

 //• ') forming the dorsal walls of the orbits. These plates, formed by the frontals {fr '), 

 orbitosphenoids {os), and alisphenoids {ah), meet one another in the middle ventral 

 line in front, forming a sort of false floor to the anterior third of the brain-cavity 

 (figs. 11 and 12, os) ; but posteriorly a large median oval space (figs. 7 and 9, A) is left 

 immediately in front of the true basis cranii, apparently serving for the transmission of 

 the second to fifth cerebral nerves. 



Posteriorly each supraorbital ridge terminates in a well-marked postorlital ])^'ocess 

 (figs. 7 and 8, sp.o), from which a pillar of bone (figs. 7, 9, 10, 11, p. or.])) is continued 

 downwards and inwards immediately in front of the subcranial crest, finally uniting 

 with its fellow of the opposite side ; these may be conveniently called the postorhital 

 pillars. 



The subcranial crest is Y-shaped in horizontal section, its posterior moiety (figs. 7 and 

 11, h.s, pa.s) consisting of a single plate formed by the basioccipital and parasphenoid, 

 its anterior moiety of paired laminae {op.o *), which diverge in front, enclosing a wedge- 

 shaped space, and are descending processes of the opisthotics. Each of these laminae 

 is separated in front by a narrow cleft from the corresponding postorhital pillar with 

 which it articulates below; there is an oval foramen (figs. 7 and 11, £) separating it 

 below from the unpaired portion of the subcranial crest. The wedge-shaped space 

 thus enclosed, beneath the anterior part of the basis cranii, by the descending processes 

 of the epiothotics and the postorhital pillars, doubtless serves for the origin of the 

 ocular muscles. 



Anteriorly, each supraorbital ridge ends in a large, blunt, imperfectly ossified ant- 

 orbital process (p.efh). Between and in front of these, the cranium is continued for- 

 wards into the prenasal rostrum {p.n.), a median, vertical, somewhat wedge-shaped 



