OCR Text |
Show 386 DR. ST. GEORGE MIVART ON THE SKELETON OF [May 7, an angle of about 23° in P. erithacus, and at a slightly more open angle in L. flavopalliatus. In P. erithacus the lambdoidal ridge traverses the posterior surface of the skull a little below its dorso-ventral middle. Above this the occiput is rounded. This lambdoidal ridge is met (as before noted) by the posterior continuation of the ridge which bounds the temporal fossa inferiorly, and at the point of junction develops an obscurely marked prominence we have called the squamosal prominence (see fig. 2, psp). Thence a slightly marked ridge descends vertically to another small prominence or exoccipital process. From this latter another very slightly marked ridge-the occipital ridge-passes inwards and upwards till it nearly joins the lambdoidal ridge, and thus a triangular surface becomes defined. Then this slight occipital ridge continues on inwards and downwards till it meets a prominence running upwards from the middle of the dorsal margin of the foramen magnum to the lambdoidal ridge, or may descend to the margin of that foramen, and thus a second triangular surface becomes defined. Beneath the occipital ridge there is, on each side of the median occipital protuberance, a rather extensive but shallow concavity. In L. flavopalliatus the lambdoidal and occipital ridges are both about equally, and only very slightly, prominent and so close together that only a faint and narrow groove runs between them. The squamosal prominence is very slight, and the exoccipital process hardly to be detected. The median rounded occipital prominence is much more marked, and so is the concavity on either side of it, but the degree of concavity is more uniform over this concave space than in P. erithacus and is most marked just below the occipital ridge. In P. erithacus each paroccipital process is somewhat pyramidal, but may be said to present two surfaces limited by a ridge which extends backwards from the hind end of the ridge bounding laterally the basi-temporal shield to the apex of the process, and thence upwards aud slightly outwards, finally curving inwards to tbe exoccipital process. The outer surface of the paroccipital process is slightly convex transversely and very slightly concave in the opposite direction. The inner surface is very strongly convex transversely and medianly concave dorso-ventrally. The process is bent much backwards (but hardly inwards) towards its apex. In L.flavopalliatus it presents more exclusively two surfaces, the ridge which divides them from each other being sharper. It is so bent that what corresponds to the outer surface of the process in the other species here looks mainly downwards. It is also concave both transversely and antero-posteriorly. The opposite surface looks mainly upwards and is strongly convex transversely at the root of the process, but concave in the opposite direction, especially towards its apex. The process is bent rather more inwards than in P. erithacus, and very much more strongly and sharply backwards. |