DEVELOPMENT OF THE SKULL IN THE OSTEICH TRIBE. 129 



however, reached the foramen magnum below, and the sagittal suture between the 

 parietals above. It has embraced three-fourths of the anterior semicircular canal 

 within (Plate X. fig. 7, a.s.c); thus anticipating the epiotic in its function, and, as 

 it were, causing it to be both late and small : this is extremely common, and in by far 

 the larger proportion of birds the epiotic is prevented from appearing by this vicarious 

 overgrowth of the superoccipital ; this is well seen also in the Chelonians. 



The " occipito-sphenoidal synchondrosis " (Plate X. fig. 7) is nearly a line in fore- 

 and-aft extent, and a notable margin of cartilage divides the whole body of the basisphe- 

 noid from its neighbour-bones laterally, e. g. the alisphenoid and the prootic (Plate X. 

 fig. 7). The two lamellae of the basisphenoid are completely welded together, and the 

 two lateral lamellae (the basitemporals) have also coalesced with the sides of the basi- 

 sphenoid below (Plate X. fig. 7, bt.). Two small passages, connected by a groove, 

 feebly remind us of the once large pituitary space ; behind these the basisphenoid can 

 be seen growing towards the basioccipital, the basitemporals being lateral, as in the 

 Mammalia (Plate IX. fig. 4). That mass of cartilage which takes the place of Rathke's 

 "middle trabecula" (the posterior clinoid bridge) (Plate IX. fig. 1, p.cl.) runs con- 

 tinuously across the floor of the skull into the tract which at present separates the 

 alisphenoid from the prootic. This clinoid bridge is filially related to the tissue which 

 formed the most projecting angle of the skull-base in the " cranial flexure " of the early 

 embryo. In front of the " infundibulum " the ossific matter has reached the common 

 optic foramen (Plate IX. fig. 2, & Plate X. fig. 7, bs.); so that there is a large com- 

 pressed prepituitary portion already developed to the basisphenoid ; afterwards it will 

 have grown forward enough to meet and coalesce with the vertical ethmoid (p.e.), 

 although they are at present 4 lines apart. The acquisition of the basitemporals gives 

 the basisphenoid a bilobate form behind and below (Plate IX. fig. 4) ; above, the bone 

 is split beneath the clinoid bridge (Plate IX. fig. 1), and the fissure is the remnant of 

 the space in which the extreme point of the notochord neared the infundibulum : it is 

 much larger in Reptiles, and being in them deficient of cartilage, forms the " posterior 

 basicranial fontanelle " of Rathke. The, at present, rather slender " rostrum " runs 

 forwards to the middle of the alse nasi (Plate IX. fig. 2, & Plate X. fig. 7, r.bs.) ; it is 

 relatively somewhat shorter in the adult ; it is entirely ossified. The alisphenoids 

 (Plates IX. & X. a.s.) are at this stage composed only of one piece ; but the ossific 

 matter has scarcely reached the great trigeminal nerve (5), and overarches only the 

 ophthalmic (5, a) ; the anterior margin is also soft, and so is part of the supero-extemal 

 angle. Only part, however ; for here is developed the true {ichthyic) " postfrontal " 

 (Plate X. fig. 8, pf.) ; it is somewhat like the blade of a hatchet, and is a line and a 

 half across. It is best seen in the Rhea of any bird I am acquainted with, although 

 it turns up in the Emu and the Tinamou (as we shall see), in the typical " Raptores," 

 e. g. Nisus vulgaris, and in the nocturnal species also, e. g. the Bam-Owl {Strix flammsd). 

 This bone is not the homologue of the so-called postfrontal of the Reptile, which is a 

 mere postorbital scale, and a reappearance (in a feebler form) of that postero-lateral 



MDCCCLXVI. u 



