OCR Text |
Show 1899.] OSTEOLOGY OF T H E PYGOPODES. 1019 a given bone belongs, when only that portion of the skeleton, as frequently happens, comes up for determination. ii. THE SKULL OF THE ADULT. The skull of the Pygopodes resembles on the one hand that of the Alcidae and the Penguins, and on the other the Rails. Its resemblance to the two former rests chiefly upon the structure of the palate, which in all is schizognathous, and of the deep supraorbital grooves, when these are present. It can at once be distinguished from the Alcidae by the holorhinal nares, which in the Alcidae are schizorhinal, and from the Impennes by the rod-shaped pterygoids, the inflated basitemporal platform, the laminate maxillo-palatine processes, and the great width aud shallowness of the temporal fossa, when present. Its resemblance to the Rails is confined to the smaller Grebes, and in these it is very striking. The skull of the Grebe, however, is always to be distinguished from that of the Eail by the conspicuous development of a cerebellar prominence, similar to that of the larger Grebes, the Divers, Penguins, and Auks. At the base of this prominence is a well-marked deepening of the posterior region of the temporal fossa which is never found in the Rails, where the fossa is only barely indicated by a very shallow depression. The Occipital Region.-The occipital condyle is more or less reniform in the Grebes and hemispherical in the Divers, though even here the flattened upper surface is slightly hollowed. The form and development of the paroccipital processes resemble those of the Penguins; they pass upwards into the lambdoidal crest and forwards into the squamosal prominence. In the smaller Grebes these processes are but feebly developed, being represented only by small and somewhat inflated bosses laterad of the base of the foramen magnum. In the Grebes, caudad of the inner end of the lip or interior free border of the process is a deep groove wdiich is not present in the Divers. The supraoccipital is not pierced by lateral fontanelles, but there is a small median foramen above the foramen magnum in the Divers ; this is wanting in the Grebes, and the cerebellar dome-formed by this bone-is marked by a more or less well-defined median vertical ridge, or low keel, forming a supraoccipital crest, differing in this respect from both Penguins, Petrels, and Auks. This crest joins the median sagittal crest, dividing the temporal fossae, at the lambdoidal ridge. The squamoso-parietal wdngs, in the Divers, rise in the form of sharp lateral ridges for the whole height of the skull as in many Penguins, terminating in the middle line in a more or less diagonal expansion, which passes forwards into the median sagittal crest. The free edges of these wings give a sharply defined crescentic outline to the skull when seen from behind. These wings, as in the Penguins, occupy the position of the lambdoidal ridge. |