OCR Text |
Show 1 030 MR. W. P. PYCRAFT OS T H E [Dec. 19, sellae. This arrangement suggests an ossification of the basisphenoid from three centres-a median longitudinal, running forward to lodge the pituitary fossa, and two lateral wings. This is a point for further investigation on fresh material. The basisphenoid is bounded laterally by the alisphenoid, postero-laterally by the prootic, and posteriorly by the basioccipital, and anteriorly by the presphenoid. The alisphenoid has partly fused with the orbital process of the frontal, but is otherwise at present free. Externally it is roughly circular in form. The posterior convex border follows the outline of the concave anterior border of the squamosal, but the two borders do not as yet even touch. Neither has the ossification of its ventral border extended downwards as far as the parasphenoid. The orbito- and presphenoids have not as yet begun to ossify. They are represented only in the dried skeleton by a thin transparent sheet of tissue-the remains of the original cartilage. The mesethmoid is almost completely ossified and subcrescentic in form, its convex border forwards. It extends vertically from the parasphenoid below to the nasal above. The quadrate has not yet assumed its fully adult form, the distal end of the orbital process being only cartilaginous and relatively shorter than in the adult. The columella is represented only by its base, which is ossified; the stapedial rays are not distinguishable in the dried skull. The articulare can still be distinguished as a separate element. b. Membrane-bones. The parietal externally is oblong in form, with its borders nearly straight and at right angles one with another. Immediately above the antero-ventral angle of its anterior border it is overlapped by a tongue-shaped process from the frontal. Its inferior border rests upon the dorsal border of the squamosal, than which it is a trifle broader. Its posterior or hinder border ventrad abuts against the epiotic, which at this point is cartilaginous, and dorsad forms a harmony suture with the supraoccipital. Internally the angle formed by its hinder and ventral borders is cut off from the inner surface of the skull by a portion of the pro- and epiotic bones. The frontal has its hinder border nearly straight. Just above its postero-ventral angle it sends backwards a slight linguiform process to overlap the parietal. Its outer, ventral border caudad extends downwards to within a short distance of the level of the squamosal, cephalad it sends downwards a small orbital process to overlap the alisphenoid. The squamosal seen externally is almost quadrate in form, but widest along its dorsal border, which, like the remaining three, is slightly hollowed. It articulates by a close harmony suture with the parietal. In the hollow of its hinder border there is a small osseous nodule representing a portion of the opisthotic. The squamosal is almost entirely excluded from the cranial |