902 jra. w. p. PYCRAFT ox THE [Dec. 1 3, 



lie two bony ridges converging in the middle line to meet over 

 the parasphenoidal rostrum. In all but Spheniscus there is a 

 more or less well-marked precoudylar fossa. The two bony ridges 

 (right and left) along the anterior border of this region represent 

 what, earlier in life, formed the free edge of the basitemporal plate 

 of the parasphenoid (p. 970). From the alisphenoid wings of the 

 parasphenoid there has grown downwards a thin plate of bone to 

 fuse with the sometime free edge just referred to. Thus tlie 

 Eustachian grooves become converted into tubes. On breaking 

 away the wall of this tube, a second smaller tube is found immedi- 

 ately above it. This also is formed in the parasphenoid and lodges 

 the internal carotid arteiy, on its way to pass into the pituitary 

 fossa. The larger, outer tube opens immediately behind the 

 quadrate and forms the external auditory meatus. 



Each mammillary tubercle is separated from the paroccipital 

 process lying behind, and without, by a wide groove, at the bottom 

 of which lie the foramina for the vagus and condyloid nerves. 



The parasjjJienoidal rostrum takes the form of a slender curved 

 rod, supporting the presphenoid and mesethmoid, and terminates 

 at a point correspouding with a section through the skull at the 

 lachrymals. Eemnants of the anterior and posterior basicranial 

 fontanelles ^ not infrequently occur, the latter being more or less 

 concealed by the basitemporal plate. 



The Lateral Aspect of the Cranium (PL LX. figs. 1-3). — The 

 tympanic cavity is a tubular opening lying behind the articular 

 end of the quadrate ; it is bounded behind by the paroccipital 

 process, above by the squamosal prominence, and mesially by the 

 prootic and occipital bones. 



The squamosal prominence is formed by a lateral outgrowth of 

 the base of the squamosal immediately above its articulation with 

 the quadrate. It forms a sloping floor to the posterior region of 

 the temporal fossa. 



The tempjoral fostsa can best be understood by a careful study 

 of its form and size in Catarrhactes (Pis. LIX., LX. figs. 5 & 2) or 

 Pijr/oifcelis (PI. LIX. fig. 2). In these it is represented by a 

 shallow horseshoe-shaped fossa lying between the postorbital 

 process and the squamoso-parietal wrings already described (p. 968). 

 Its outline is defined by a raised surface, representing the extreme 

 limit of attachment of the temporalis muscle ; from the post- 

 orbital process it sweeps upwards, backwards, and downwards to 

 terminate on the squamosal prominence. The greater part of the 

 fossa rests upon the convex wall of the cerebral dome ; posteriorly, 

 from the squamosal prominence to its vertex, it is much deepened. 

 From the apex of its semicircular outline there runs a well-defined 



' In my paper on the Osteology of the Steganopodes (15), in describiug 

 the skull of Fregata, 1 mistook this anterior basicranial fontanelle for the 

 Eustachian aperture. The Eustachian tubes in this genus are represented by 

 grooves i'ormed by the free edge of the basitemporal plate. The " traces of 

 the Eustachian tubes" in the skull of Sida, referred to in this paper, are, as in 

 the case of Fregata, remnants of this embryonic fontanelle. 



