50 PEOP. W. K. PAEKER ON THE 



developed in front of the large, hollow occipital arch ; the alisphenoids (Pis. VII., 

 VIII. fio'. 2, al.s.) are large and simply postorbital in position ; whilst the basi- 

 sphenoid, dominated by the auditory organs, and underfloored by the huge para- 

 sphenoid, is one of the largest and most complex structures to be seen in the whole 

 skull. The anterior sphenoid (p.s.) is the hinder half of the interorbital partition ; it 

 is partly segmented from the meso-ethmoid, even in the earliest of my stages, by an 

 oval fenestra (PI. VII., i.o.f.). The upper part, which overhangs the large optic 

 passage (ii.), as a rule, has scarcely any development of cartilaginous lips representing 

 the orbito-sphenoids {o.s.). The only bird in which these are at all well developed, even 

 in the early embryo, is the African Ostrich (Phil. Trans. 1866, pi. vii. figs. 1-3, o.s.). 

 In Opistliocomus they are well developed for a bird (PL VII., o.s., and Pi. VIII. fig. 2, 

 O.S.), and come nearest the Ostriches of any I have yet seen (see in the chick of the 

 Common Fowl, Phil. Trans. 1869, pi. Ixxxiii. figs. 2 & 4). In the 3rd stage the peculiarity 

 of the ethmoidal region of a bird is well shown ; the pars perpendicularis {p.e.) has 

 already a large reniform osseous centre (PL VIII. fig. 2, p.e.) which will ultimately reach 

 twice as far back to ossify the cartilaginous crista galli {cr.g.), and also, below the 

 interorbital fenestra, will grow some distance backwards to meet the feeble presphenoid 

 {'p.s.) and the enormous basisphenoid [b.s.). This chondrocranium, so different from 

 that of a Mammal, on the one hand, and from a Batrachian, on the other, already, in 

 the 3rd stage, shows the beginning of that character which separates the skull of the 

 Carinatse, not only from that of other Vertebrata, but also from that of the Katitse 

 themselves. This is the secondary segmentation of the vertical trabecular wall, so that 

 the septum nasi in front is quite, or nearly, separated from the perpendicular ethmoid 

 behind. In this modification, 0])isthocomus agrees with the Carniatas generally ; and 

 in the ord stage (PL VIII. fig. 2) the separation has taken place to an extent equal to 

 what I have found it in an adult Tinamou (see Phil. Trans. 1866, pi. xv. fig. 8, c.f.c.y. 

 In the earlier stage the whole prochordal tract is continuous and only loses its vertical 

 crest wliere that part is not wanted, namely, in front of the nasal labyrinth ; thus the 

 intertrabecula runs on as a free bar in front of its crested part. But in the 3rd stage 

 (PL VIII. tig. 2, i.tr.), under the notch just mentioned, the fore part of the inter- 

 trabecula is already separated from that which thickens the dividing wall at its base ; it 

 is even now undergoing degeneration, and will disappear entirely after a time. This 

 is only one of the many prenatal transformations to be seen in the embryo bird, all 



■ I have spoken before of the remarkahle isomorphism existing between the culminating Fishes (Teleostei) and 

 the culminating forms of the Sauropsida, seen in the extreme mobility of the jaws and palate. This mechanism 

 is obtained in the former by the hyostylio condition of the pier of the mandibular arch, the palato- 

 quadrate beiug swung on the hyo-mandibular and sympleotic. But there is no segmentation of the cranial 

 axis in front ; that part is extremely short ; and the large dominating premasiUaries ride over it, and in 

 most cases throw the maxillaries back, as the edentulous " ossa mystaoca." I may state that in such a 

 bird as the Cock of the Woods {Tetrao urogcdlus) those bones are extremely like their counterparts in the 

 Teleostei. 



