OF COLYMBUS TORQUATUS. 139 



quadrilateral pyramid, of which the tip of the bill is the apex, the occipital bone chiefly 

 the base, and the superior, inferior, and two lateral aspects respectively the four sides. 



The base presents rather a pentagonal outline. Superiorly, the two occipital crests con- 

 verge at an angle on the median line, terminating below on each side with the mastoid 

 processes. The lateral boundaries of the pentagon are formed by the pedicellate bones, 

 the tempero-maxillary articulation, and the surface at the end of the inferior maxillary 

 bone for the attachment of the digastricus. The fifth side of the pentagon is an imaginary line 

 drawn from the posterior extremities of the rami of the lower jaw. Exactly in the centre 

 of this pentagon is the semilunar condyle for the cranio-vertebral articulation ; just above it 

 is the large oval foramen magnum, between which and the occipital crests lies the flat, de- 

 pressed "spinous" plate of the occipital, — the two sides of which rise into a longitudinal 

 ridge at their union along the median line. On either side, the mastoid processes project 

 conspicuously. Beneath these, and between them and the ossa pedicellata, are situate the 

 internal organs of hearing. The space around the foramen magnum is pierced by several 

 foramina for the transmission of nerves. 



The superior aspect of the skull is bounded posteriorly by the occipital ridge, laterally 

 by the parietal bones, the superior margins of the orbits, and the sides of the intermax- 

 illary. The zygomatic processes of the temporals are the first lateral protuberances from 

 behind forward ; between these and the post-orbital processes, the crotaphyte depression is 

 deep and well marked. The post-orbital processes are long and acute, the width between 

 their apices representing the greatest lateral width of the skull. Anterior to these, the mar- 

 gins of the superior aspect of the skull are deeply concave, by the indentation formed by the 

 upper margins of the orbits ; the ante-orbital processes form a slight concavity on each side 

 just posterior to the fronto-maxillary suture. The tapering intermaxillary bone forms the 

 remainder of the boundary of this aspect of the skull ; its superior surface is occupied as 

 far forward as the post-orbital processes, by the large, deep " temporal " fossa, formed chiefly 

 however, by the parietal bones. Along the margins of the orbits lie the two lono- semilunar 

 depressions for the nasal glands, separated by a median ridge from each other, their divari- 

 cation posteriorly leaving between them and the temporal fossa) a central elevated, smooth 

 lozenge-shaped space. At their termination the glandular depressions are perforated 

 with oblique oval foramina, which transmit their ducts through to the nasal cavities. The 

 curved fronto-maxillary suture extends transversely across from side to side, its convex- 

 ity presenting backward ; the smooth nasal bones come next ; and, taking its origin from be- 

 tween them, and stretching from the frontal bone to the tip of the bill, is the intermaxillary. 



The inferior aspect is bounded laterally and apically by, and contained chiefly within, 

 the two sides of the lower jaw r , which divaricate from the symphysis, and extend the 

 whole length of this aspect of the skull. Posteriorly, on the median line, lies the basilar 

 portion of the occipital ; just anterior to it, the spheno-occipital anchylosis, from which runs 

 forward in the median line the elongated vertical spine of the sphenoid, to be received into 

 the cleft of the vomer, and there anchylosed with the ethmoid. On either side of this spine 

 are the pterygoid bones, running backward from the pterygo-palatal articulation, and diverg- 

 ing outwards to join the head of the pedicellate bones. More anteriorly, the flat, elongated 

 palate bones extend the whole distance from the pterygoid to the bases of the superior 

 maxillary, lying on either side of the median line, the vomer being interposed between 

 them, and articulated with them by its posterior bifurcated extremity. As far as the an- 

 terior extremity of the vomer, the spaces leading up into the orbits and nasal passages 



MEMOIRS BOST. SOC. SAT. HIST. Vol I. Pt. 2. 30 



