] 020 MR. W, p. PYCRAFT ON THE [DeC. 19, 



In the Grebes, the squamoso-parietal wings do not attain to tlie 

 height of the skull. In the larger species they cease abruptly at 

 about halfway, a thin raised ridge running upwards from this 

 point to the sagittal crest representing the lambdoidal ridge. 

 In the smaller Grebes the squamoso-parietal wings are feebly 

 developed. As in the larger species, the lambdoidal ridge is 

 represented by a thin raised line, terminating at the sagittal 

 crest. 



The Roof of the Cranium. — The parietal region in the Divers 

 and larger Grebes is impressed by wide but shallow temporal 

 fossae, divided, in the fully adult bird, by a narrow sagittal crest 

 (Plate LXXII. fig. 1). It is interesting to note that in a nearly 

 full-grown skull of Colymhus septentrioualis in the Museum Col- 

 lection this crest (Plate LXXII. fig. 2) is represented by a broad 

 plate of bone, whilst in an immature O. ylacialin, apparently a 

 little younger than that of C. sej>te>itrionalis, inasmuch as some 

 sutures are yet distinct, the sagittal crest is as sharply defined as 

 in the adult. 



In the larger Grebes the sagittal crest is sharply defined, as are 

 the temporal fossae. In the smaller species the anterior boundary- 

 line of the temporal fossa is barely visible, posteriorly the fossa is 

 moderately deep. The form of the fossa differs from that of the 

 larger species and Divers, in that it is relatively shorter from 

 before backwards, the cerebellar region of the skull only slightly 

 projecting backwards beyond the cerebral. In the former the 

 backward extension of the cerebellar prominence is very marked, 

 more so than in the Penguins and Petrels. 



The supraorbital grooves of the frontals in the Divers are 

 separated by a median knife-like edge ; externally, they are bounded 

 by a broad supraorbital ledge the free edge of which is flattened, 

 as in many Penguins. Anteriorly, the supraorbital ledge fuses 

 with the posterioi'-dorsal limb of the lachrymal on either side. 

 The anterior inner border of the ledge, immediately behind the 

 lachrymal, is pierced by a large foramen for the passage of the 

 lachrymal duct. 



In the Grebes, the supraorbital region of the frontals is marked 

 by a wide shalloAv median furrow; supraorbital grooves can hardly 

 be said to exist, being represented only by a faint excavation along 

 the free edge of the frontal. 



The supraorbital ledge, in the Divers, posteriorly combines with 

 the alisphenoid, to form a prominent postorbital process. In the 

 Grebes, this process is only very feebly developed ; moreover, this 

 region of the skull differs markedly in the two forms, in that, in 

 the Divers the postorbital process is continued forwards as the 

 supraorbital ledge, whilst in the Grebes the region in front of 

 this process is marked by a shallow depression for the insertion 

 of the muscles of the jaws, ^climojphorus seems to be the only 

 Grebe to which the above remarks do not apply ; in this genus the 

 periphery of the fossa lodging the muscle is produced outwards 

 into a broad shelf-like emarginate postorbital process. 



