MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^NICEPS REX. 283 



and a posterior expanded pair that form the anterior wall of the tympanic cavity and 

 the superior and anterior boundary of the Eustachian tubes. The anterior or inner pair 

 are represented in Balteniceps by two bevelled articular surfaces on the sides of the 

 great rostrum near its middle. We shall give some tables showing the modifications 

 of these parts in various birds relatively to the pterygo-palatine apparatus. In front 

 of these facets the basi-sphenoid of Balseniceps becomes gradually thinner, ending in a 

 quadrate wedge-like process two lines anterior to its proper preceding homologue, the 

 descending plate of the pre-sphenoid ; to this plate the rostrum of the basi-sphenoid is 

 anchylosed. Behind the articular facets the rostrum expands, at first gradually, form- 

 ing a thick beam of bone three lines across, and then it suddenly expands on each side 

 into the elegant, conchoidal, anteriorly incurved ecto-pterapophyses, the counterparts 

 of the marginal wings of the basi-temporal. This element has most substance at the 

 part where its tables and diploe pass into the descending plate of the orbito-sphenoids, 

 just below the optic foramina. 



That this is a most important basi-cranial element in birds is seen from its connexion, 

 which must be studied in immature specimens. In the Chick, on the eleventh day of 

 incubation, more than the posterior third of the rostrum is ossified, and this ossific 

 centre, convex below and scooped above to receive the cartilaginous plate of the pre- 

 sphenoid, bifurcates behind, each posterior wing-like moiety lying just in front of, and 

 above, the basi-temporals. This single mesial osseous centre appears to be identical with 

 the distinct so-called pre-sphenoidal ossification spoken of by Dr. KoUiker (Berichte 

 von der Koniglichen Zool. Anstalt zu Wiirzburg, 1849, p. 40) and Huxley (Croon. Lect. 

 p. 11). 



lu the Chick, on the fourteenth day, this ossification of the true basi-sphenoid has 

 already occupied all but the tip of the rostrum, and posteriorly has grown upwards 

 in the direction of the ali-sphenoids and petrosals, and backwards towards the basi- 

 occipital. 



On the sixteenth day, however, these parts and their relations can be still more 

 advantageously seen ; for by this time the anterior cartilaginous tip is very small, whilst 

 the posterior end of this large and most elegant osseous centre has reached the basi- 

 occipital on the mid-line of the cranial floor, about a line behind the sella turcica, thus 

 entirely excluding the basi-temporals. The groove on the upper surface of the gradually 

 widening rostrum is bounded behind by the anterior wall of the sella turcica with its 

 middle and lateral ' clinoid processes,' whilst opposite these processes the basi-sphenoid 

 expands into the large ecto-pterapophyses, on the upper part of which the ali-sphenoids 

 are attached ; nearly the anterior half mesially being occupied by the deep ' sella,' the 

 posterior margin of which is even and smooth, whilst its fundus is perforated by a large 

 foramen (in adult birds) which communicates with the internal carotids. Behind the 

 ecto-pterapophyses the basi-sphenoid contracts, its sigmoid converging margins articula- 



