272 MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^ENICEPS REX. 



in such a manner as to slope downwards anteriorly ; at its middle is placed the single 

 hemispherical condyle, which is directed backwards, and slightly downwards, to form 

 with the atlas a ' procoelian ' articulation. Above this condyle is the ' foramen magnum ' 

 (fig. 3,fm), in shape slightly pentagonal, and nearly half an inch in diameter. 



The supra-occipital region is one inch across, and besides its tiny mesial crest, has a 

 pair of supero-lateral eminences (PI. LXV. figs. 1, 3, 6, ep), the centre of the ' epiotic ' 

 pieces of the young bird : these elevations contain part of the superior and external semi- 

 circular canals. The epiotic eminences are separated from the upper mastoid eminences 

 (PI. LXV. figs; 1,3, 6, m) by a depression of the marginal ridge, and by the vascular 

 grooves which burrow in the occipital bone. 



The outer half of each side of the upper margin is occupied by the upper mastoid 

 eminences ; they are therefore external to, and somewhat below, the epiotics, and are 

 separated from the so-called par-occipital processes by a deep fossa. These upper 

 mastoid projections help to wall-in the external semicircular canals and to bound the 

 temporal fossa behind : they answer to the upper part of the mastoid of the Turtle 

 (the ' par-occipital' of Owen, 'occipital externe' of Cuvier). 



The elegantly curved ' par-occipital' processes (PI. LXV. figs. 1, 3, 7, eo & pro) 

 pass downwards, forwards, and inwards : passing insensibly into the basal region, they 

 bound the ear-cavity behind and below, and contain part of the external and anterior 

 semicircular canals. The upper margin of these processes laterally is the lower 

 boundary of the temporal fossa, the two regions being separated by a ridge, below 

 which ridge the ' par-occipital ' forms a beautiful shell-like process, the anterior sigmoid 

 margin of which very nearly reaches the huge 'osquadratum' (PI. LXV. figs. 1, 3, 6, 7,q). 

 Below and in front of this part the substance of the bone contracts, and passes on to 

 join the delicate crescentic ' pterapophysis,' which forms the boundary of the basi- 

 temporal, a bone to be described presently. 



The basi-occipital and basi-temporal regions are, however, separated laterally by a 

 notch, mesiad of which is a large reniform passage ; the bridge over this passage being 

 the connecting link between the basal part of the occipital and sphenoidal sclerotomes. 

 This very pretty reniform opening is apparently the external outlet of the vagus nerve 

 (PI. LXV. figs. 3 & 7, vg). The basi-occipital region is of small antero-posterior extent, 

 and lies directly between these foramina ; it is slightly scooped and convex all round the 

 condyle, and is perforated by a number of vascular openings. The condyle (fig. 3, c) 

 is one-third of an inch in diameter, the entire breadth of the occipital bone is more than 

 two inches, the height of the supra-occipital above the foramen magnum is two-thirds 

 of an inch, and the wing-like par-occipitals are one inch deep. 



Internal to the foramen magnum the bony labyrinth projects mesiad on each side, 

 these projections being the line of anchylosis of the lateral occipitals (ex-occipitals of 

 Professor Owen) with the largely developed petrosals. The external union of these 

 elements is indicated by the fenestra ovalis (fig. l,/o) and rotunda (the latter being of 



