1899.] OSTEOLOGY OF THE PYGOPODES. 1025 



The mesencephalic fossa agrees with that of the Tubinares, in 

 that the groove for the sinus trausversus branch of the vena 

 cephalica posterior (pp. 1021-2) appears as a deep tunnel excavated 

 out of the inner wall of the skull. It runs upwards, backwards, 

 and downwards, following the curve of the anterior semicircular 

 canal, finally piercing the wall of the supra-occipital tunnelwise, 

 leaving the skull by a small aperture on either side of the foramen 

 magnum, but much lower down than in the Impennes or Tubinares. 

 The Pygopodes differ from the two last mentioned groups in that 

 this cephalic vein enters the skull through the trigeminal foramen, 

 and not by a separate aperture. The floor of the fossa bears a 

 deep groove for the orbito-nasal nerve. 



The pituitary fossa differs conspicuously both from that of the 

 Impennes and of the Tubinares by its peculiar shallowness. In 

 the Colymbi it forais a moderately deep pit sloping gently back- 

 wards ; but in the Podicipides it is represented only by a very 

 slight oblong depression, bounded on either side by a strong ridge 

 forming a tunnel for the abducent nerve. This pituitary ridge, 

 from the point where the nerve enters, is continued upwards and 

 outwards, to terminate at the groove for the cerebral vein, already 

 described. This second ridge forms the posterior boundary line 

 of the mesencephalic fossa. There is a well-developed dorsum 

 sellce and prepituitary ridge in the Colymbi, the latter flattened to 

 form an optic platform. The pre-optic, as usual, passes on either 

 side into the tentorial ridge. 



In the Podicipides there is no dorsum sellce, the internal carotid 

 apertures opening directly on to the floor of the fossa. 



The op)tic foramen in the Colymbi is bounded in front by a 

 vertical plate of bone, being that part of the interorbital septum 

 bounding the interorbital fenestra posteriorly. This plate is 

 wanting in the Podicipides. 



The cerebral fossce in the Divers and Grebes closely resemble one 

 another, they both agree in that this region of the brain -cavity 

 is greatly depressed dorso-ventrally, as in the smaller Tubinares. 

 The tentorial ridge is sharply defined, particularly so in the Divers. 

 The bony falx, which dips diO\Yu between the pallia! fissures of the 

 brain, is not so strongly marked as in the Tubinares. There is 

 only the faintest indication of the bony ridge marking the position 

 of the Sylvian furrow such as is found in the skull of Diomedea, 

 and this is entirely dorsal, and not lateral as in Diomedea. Again, 

 the tentorial ridge of the Pygopodes and smaller Tubinares lies 

 horizontally to the long axis of the skull ; whilst in Diomedea, for 

 instance, it is almost vertical in position. 



The olfactory fossce, like those of the Impennes, are extremely 

 small, and pass insensibly into the cerebral fossae behind ; wherein 

 they stand strongly contrasted with the large tubular chambers 

 of the Tubinares. They are divdded in the middle line, forwards, 

 by a small knife-like crista-gcdli. In the Tubinares, it will be 

 remembered, this crista-galli is columnar. 



