1024 ME. W. p. PYCEAFT ON THE [DeC. 19, 



reduced to a spine-like process dividing two greatly reduced 

 passages for (he olfactory nerA^es, the crura lying caudad of the 

 crista, aud iiot passing on either side. 



The aliethmoids are the only ectoethmoidal ossifications iu 

 either the Impennes, Tubinares, or Pygopodes. They constitute 

 the antorbital plates. In the Colymbi, when present, they 

 resemble in form those of the Tubinares — plates of bone jutting 

 out from the mesethmoid to the lachrymal, sloping obliquely 

 forwards and downwards. They appear, however, in the Colymbi 

 to be but rarely ossified, and never so well developed as in the 

 Tubinares. In the Podicipides their true nature is well seen. 

 Here, they appear as scroll-like bars of bone running continuously 

 backwards, downwards, and inwards from the expanded dorso- 

 lateral plates of the mesethmoid itself, to pass eventually into the 

 middle of its posterior border. 



The olfactory chamber is of comparatively small size. 



The lachrymal in the Podicipides is free, small in size, roughly 

 semilunar in shape — with the convex border forwards — and 

 apparently a disappearing structure. It articulates by its 

 superior limb with the outer border of the nasal bone. Though 

 conspicuous from a lateral view, it is scarcely if at all visible from 

 the dorsal aspect of the skull. 



In the Colymbi, the lachrymal is roughly of the same shape as 

 in the Podicipides ; but it differs therefrom, markedly, in several 

 points. Its superior limb, as in the Grebes, articulates with the 

 nasal ; but it is free only in the young bird, later it becomes 

 indistinguishably fused with that bone ; moreover, it sends back- 

 wards a long spur to fuse with the supra-orbital ledge, and to 

 enclose with its aid a passage for the lachrymal duct, as in many 

 Alcidae. Its lower limb, at its free end, is more or less markedly 

 sigmoidally curved. It is, on the whole, a larger and stronger 

 bone than in the Grrebes. 



The Cranial Cavity. — The metancephalic fossa of the Pygopodes 

 is relatively both longer and shallower than in the Impennes and 

 Tubinares, but is deeper in the Grebes than in the Divers. The 

 vagus foramen occupies relatively the same position as in the two 

 last mentioned groups ; the two condyloid foramina, similarly, lie 

 mesio-caudad of this. 



The internal auditory meatus lies immediately under the mouth 

 of the floccular fossa and in front of the vagus foramen. The 

 abducent foramen pierces the anterior border of the fossa, passing 

 on either side of the pituitary fossa, and emerging in the Grebes 

 within the rim of the ventral border of the optic /oramen, and in 

 the Divers caudad of the foramina for the oculomotor nerve and 

 internal ophthalmic artery. 



The cerebellar fossa agrees wdth the Tubinares, and differs from 

 the Impennes, in its greater relatiA'^e size. It lacks, however, the 

 transverse system of grooves and ridges representing the cere- 

 bellar sulci and gyri so marked a feature in this region of the skull 

 in the Tubinares. 



