1899,] OSTEOLOGY OF THE 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 supra- 

 orbital grooves, when these are present. It can at once be 

 distinguished from the Alcidae by the holorhinal nares, which in 

 the Alcidte are schizorhinal, and' from the Impennes by the rod- 

 shaped pterygoids, the inflated basitemporal platform, the laminate 

 maxillo-palatine processes, and the great width and 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 Grrebe, however, is always to be distinguished 

 from that of the Rail 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 

 Avell-marked deepening of the posterior region of the temporal 

 jossa 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 hollo\\'ed. The 

 form and development of the paroccipital processes resemble 

 those of the Penguins ; they pass upwards into the lambdoidal 

 crest and forwards into the sqiiamosal 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 which is not present in the Divers. The supra- 

 occipital is not pierced by lateral fontanelles, but thei'e 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 wings, 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. 



