1898.] OSTEOLOST OF THE IMPENNES. 969 



processes as far as the level of the anterior border of the lachrymal ; 

 and backwards, to a point corresponding with the level of a line 

 drawn through the articulation of the quadrate with the squamosal. 

 They leave a small fontanelle in the middle line at their junction 

 with the parietal. The outer free border of each is sharply 

 depressed and slightly hollowed when seen from without ; this 

 groove runs forwards to skirt the outer border of the nasals. The 

 form which this groove ultimately acquires is of considerable 

 importance for taxonomic purposes. (The nature and extent of 

 these changes can be seen at a glance by comparing PL LXI. Sg. 2 

 with PI. LIX. fig. 2.) Thus, in the nestling the supra-orbital 

 groove is represented by a shallow depression or hollowing out 

 of the whole outer border of the frontal — the inner wall of the 

 groove of the adult. The outer wall of the adult gradually arises 

 from the inferior border of this inner wall, and eventually assumes 

 the form of a huge overhanging ledge of bone with a wide, flattened, 

 dorsal rim along its free edge. The condition of the groove in the 

 adult skull of Aptenodt/tes affords a more or less intermediate stage 

 between these two. It is interesting to note that Watson, in 

 his memoir published in the ' Challenger ' Eeports (vol. vii. p. 6), 

 described and figured the skull on which this description is based, 



and remarked that in " Apienodytes and Pyc/oscelis this 



ledge of bone does not exist " 



The basioccipital, seen ventrally, is bounded on its outer sides 

 by the exoccipital, and in front by the basitemporal. Posteriorly 

 it is rounded off to form the main body of the occipital condyle. 

 Seen dorsally, it is bounded in front by the basisphenoid, laterally 

 by the prootic, opisthotic, and exoccipital. At this stage, in a very 

 young nestling of Catarrhactes chrysocome, all these boundaries 

 are clearly defined in cartilage, save that between the opisthotic 

 and prootic. Soon after this all traces of these limits become 

 obliterated. 



The exoccipital is for some considerable time separated from the 

 squamosal by a wide gap filled in by cartilage, through which 

 the prootic has thrust itself (Plate LXI. fig. 3). Its supero- 

 internal dorsal border abuts against the epiotic and supra-occipital 

 bones, both of which can be readily distinguished. Below, and 

 externally, it develops a short paroccipital process, which, how- 

 ever, never acquires a large size : separated from this by a wide 

 groove is developed the mammillary process, abutting against the 

 basioccipital at its junction with the basitemporal. Seen from 

 within, the exoccipital appears as a small triangular area of bone 

 wedged in between the opisthotic and the basioccipital and 

 contributing to form the foramen magnum. 



The hasisphenoid is not visible externally, being concealed by 

 the underlying basitemporal plate and its parasphenoidal rostrum. 

 Internally, it is bounded, laterally, by the prootic and alisphenoid, 

 posteriorly by the basioccipital, and anteriorly by the pre- 

 sphenoidal cartilage. Its dorsal border is hollowed out to form 

 the ventral segment of the optic foramen. In front of the pituitary 



Pboc. Zool. Soc— 1898, No. LXIV. 64 



