1022 MR. W. p. PTCBAFT ON THE [DeC. 19, 



The tympanic cavity in the Grebes is uot so sharply defined as 

 in the Divers, owing to the slighter development of the ali- 

 sphenoidal wing of the parasphenoid ; in other respects, however, 

 it closely resembles that of the Divers, 



The squamosal prominence is only concerned with the suspension 

 of the quadrate and does not, in addition — except very imperfectly, 

 and only in the Divers — form the roof of a temporalis recess, as 

 in the Tubinares and the Impennes. The paroccipital processes 

 are well developed in the Colymbi, forming, as in the Impennes, a 

 backward continuation of the squamosal prominence ; in the 

 Grebes (Podicipides) these processes are feebly developed, being 

 represented only by an abruptly truncated lamina of bone. 



The temporal fossce. — In the Colymbi (Divers), and in the larger 

 Podicipides (Grebes), the temporal fossae are very wide and shallow 

 and sharply defined. They are separated in the mid-dorsal line 

 only by a sharp median sagittal crest. Within the confines of 

 the fossa, on each side, are defined the limits between the cerebral 

 aud cerebellar regions of the skull, the squamoso-parietal wings 

 running transversely across the cerebellar dome, as in the Petrels, 

 aud not, as in the Penguins, traversing the boundary line between 

 these two regions. In the smaller Grebes, e. g. Tachyhaptes, the 

 temporal fossa is only faintly defined, moreover it is almost 

 entirely confined to the cerebral, and extends scarcely at all on to 

 the cerebellar dome. This is due, uot to any essential difference 

 in the form or position of the fossa, but to the extremely slight 

 development of the last-mentioned dome. 



The trigeminal foramen, in the Divers, lies immediately outside 

 the inner wall of the mouth of the fossa representing the 

 temporalis recess. There is no foramen for the sinus transversus 

 branch of the vena cepJialica posterior above this as in the 

 Penguins and Petrels. In the Grebes the trigeminal and the 

 venous foramen immediately above ai-e confluent ; it pierces the 

 Avail of the alisphenoid immediately in front of the articular 

 surface for the head of the quadrate and below the anterior 

 border of the squamosal prominence. It lies relatively further 

 forwards than in the Di\'ers, inasmuch as, in the Grebes, this 

 foramen and the anterior border of the squamosal prominence lie 

 only a short distance behind a vertical line passing through the 

 postorbital process ; in the Divers both these, points lie far behind 

 this line. 



The orbits, in the Divers, are overarched by the supraorbital 

 ledges, the postorbital process bounding them posteriorly and the 

 lachrymal anteriorly. The perforated iuterorbital septum forms 

 a mesial partition-wall. 



In the Grebes the supraorbital ledges and postorbital processes 

 are wanting, and the lachrymal is very small, so that the orbit in 

 this group is iiot neax'ly so well defined. An osseous iuterorbital 

 septum is wanting ; in the Divers a large extent of the middle 

 region is supplied by membrane, so that in the dried skull the 

 seplum is largely fenestrated. 



