MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^NICEPS REX. 301 



boundary between it and the surrounding bones is lost in a profusion of diploc, delicate 

 as the veins of a leaf, and light as a downy feather. On each side of the shallow groove 

 between the posterior edge of the coalesced pterapophyses there is a considerable pneu- 

 matic foramen, and from each spur-like point outside these passages there is a delicate 

 ridge, which converges towards its fellow at the base of the vertical keel of the ethmoid, 

 and then expanding forms on each side a wing-like crest of bone, which lies behind and 

 strengthens the diploe of the pterapophyses as they pass upwards and a little outwards 

 to coalesce with their roof-bone just within and behind the nostril. The lacrymals con- 

 tribute of their substance to strengthen the bone at the same point ; but in front of the 

 incurved lacrymal plates, inside the pre-maxillary wall, outside the ethmoid, and above 

 the up-tilted zygomatic plates of the maxillaries, there is on each side, in the dry skull, a 

 large triangular space which opens freely into the orbits behind, and into the proper 

 nasal fossae above. Below these maxillary zygomatic plates there is another open space 

 about half the size of the former ; it is open behind ; its floor is the extreme end of the 

 palatal plate of the maxillary outside, and the broad origin of the palatine mesiad ; 

 whilst it is bounded internally by diploe common to the maxillary and the ethmoidal 

 pterapophyses, and externally by the posterior angle of the maxillarj'. 



We shall speak of the development of the facial sclerotome in birds, and of its 

 general relations, after the maxillary and the ' zygoma ' have been described. 



Maxillary. (PI. LXV. figs. 6 & 7, m .r.) 



At its origin the maxillary is broad, and its inner portion is principally composed of 

 diploe, which passes insensibly into that of the ethmoidal pterapophyses mesiad, whilst, 

 above and below, it arises in the fine diploe already mentioned as lying in front of the 

 two large open posterior facial spaces ; externally it is more compact, and is continuous 

 with the posterior part of the wall of the pre-maxillary in front of the maxillary angle. 

 The broad cellular origin of the maxillary soon contracts, the bone becoming com- 

 pressed into a thin obliquely descending plate of bone, which lessens rapidly in width 

 from the inner side. Whilst lessening in width it acquires a thick inner margin, and at 

 about two lines behind the anterior zygomatic joint, the external part of which is evi- 

 dently formed by the ' malar' or 'jugal,' it suddenly thickens as it is lost on the inner 

 side of the zygomatic arch : just in this swelling region of the maxillary there are some 

 pneumatic holes. The width of the up-tilted slanting portion of the maxillary is nine 

 or ten lines at first ; at an inch further backward, where it becomes lost in the malar, 

 its width is four lines. 



Zygomatic arch. (Pi, LXV. figs. 1, 6, & 7, mx, mr, qj.) 

 Where we can see the elements of the ' zygoma' in birds, we find that the zygo- 

 matic process of the maxillary forms about a third of the arch, lying within and below 

 its successor. This next piece forms about another third, and lies in a supero-external 



VOL. IV. PART VII. 2 u 



