1899.] OSTEOLOGY OV THE PYGOPODES. 1021 



Ike Base of the Skull — The basitemporal plate of the para- 

 sphenoid is markedly inflated, and has a bevelled anterior border, 

 the free edge of which, in the Divers, is overhung by a down- 

 growth from the alisphenoidal wings of the parasphenoid. This, 

 in the Grebes, by its fusion with the free edge of the basi- 

 temporal plate, forms a pair of closed tubes opening on either side 

 of the skull, behind the quadrate, and below the squamosal pro- 

 minence into the apertui'e which serves also as the mouth of the 

 tympanic cavity. The inferior and posterior of these two runs 

 transA'ersely across the skull, and forms the Eustachian tubes of 

 the right and left sides of the head. The connection with the 

 choanfe is by means of a single median aperture immediately under 

 the rostrum. The anterior runs forward as a pneumatic cavity 

 into the body of the parasphenoid to terminate beneath the level 

 of the foramen opticum. In the Divers, the foi-m of this aperture 

 is tubular, recalling that of the Penguins, the anterior wall of the 

 tube being continued outwards behind the squamosal, but in the 

 Grebes the anterior wall is deficient. 



Mammillary processes are but feebly developed ; in the Divers 

 the paroccipital notch is wide and shallow, it can scarcely be said 

 to exist in the Grebes. There is a precondylar fossa, or rather 

 groove, in the larger species of both families. 



The parasphenoid rostrum, in both Grebes and Divers, is some- 

 Avhat inflated at the base, owing to the presence of the pneumatic 

 cavity already described. 



The Lateral Aspect of the Cranium (Plate LXXII. figs. 3-6).— 

 The tympanic cavitj' has a sharply defined aperture in the Divers 

 (Colymbi), bv reason of the considerable latez'al development of the 

 alisphenoidal wing of the parasphenoid. Within its mouth can 

 be seen, distad, two large apertures, lying immediately behind the 

 alisphenoidal wing just referred to : the upper is the pneumatic 

 aperture of the parasphenoid, the lower is the Eustachian aperture ; 

 caudad, and separated by a broad column of bone, lie the fenestra 

 oralis and the fenestra rotunda. 



The temporalis recess'^, so well developed in the Steganopodes and 

 Petrels, and to a lesser degree in the Penguins, is here represented 

 only in the Divers, by a moderately deep fossa ; in the Grebes it 

 is wanting. The posterior pneumatic cavity opening downwards, 

 behind and above the fenestra ovalis, so well developed in the 

 skull of the Tubinares, is wanting in both Grebes and Divers. 



^ In my recent paper on the Osteology of the Penguins the temporalis recess 

 ■was described as "leading eventually, in the dried skull, into the cranial cavity." 

 This is quite a mistake. The correct interpretation of this is as follows : — In 

 many skulls, e. g. Puffinus, above the trigeminal there lies a second foramen 

 for the sinus transversus of the vena cephalica posterior — at times this is 

 confluent with that for the trigeminal, e.g. Divers — and both these lie im- 

 mediately outside, below and mesiad of the moutii of the recess in question. 

 In the Penguin the foramen supplementary to the trigeminal lies immediately 

 within the mouth of the recess, piercing its inner wall ; owing to imperfect 

 ossification, the mouth of the foramen in the dried skin extends upwards 

 nearly the whole length of the recess. 



Pkoc. Zool. See— 1899, Js'o. LXYI. QQ 



