124 ME. W. K. PAEKEE ON THE STEUCTUEE AND 



Struthio, " B." 



The head of the next embryo of Struthio cameltis measured 2| inches in length, but 

 although the body of the chick was about the size of that of a pigeon, yet there was 

 nearly a pound weight of unused yolk in the egg. 



At this stage the skull has increased to nearly twice the length of that which I have 

 been describing, and with this greatly augmented bulk we also have the incoming of 

 new osseous parts, as well as a great maturation of those already existing, and a 

 commenced deterioration of those quasi-larval structures which disappear during the 

 slow, gentle, but real metamorphosis which the parts of the ornithic skull and face 

 undergo. 



The angle of the occiput with the basicranial axis (Plate VIII. fig. 1) has greatly 

 lessened, and is not many degrees above a right angle, to which degree of acuteness it 

 never attains even in the adult bird. The base of each hemisphere, anteriorly, is in 

 Struthio, A. five lines, or nearly, above the true base of the skull, i. e. the lower edge of 

 the basisphenoidal rostrum ; in Struthio, B. it is seven lines, and in the adult twenty- 

 one, or an inch and three-quarters, the rostrum at that part being seven lines in depth. 

 But the brain at this part lies on the gently convex surface of the orbitosphenoids ; the 

 anterior sphenoid is thus altogether lifted up above the actual basal line of the skull, 

 the whole depth of the basisphenoid below and just in front of the optic foramina being 

 in the adult a full inch. To the student of the mammalian skull all these details will 

 appear strange enough ; but the bird's skull is a curious problem, and he who shall 

 explain it will have done much towards producing a harmony of all vertebrate crania. 



The cartilaginous investing mass in Struthio, B., has on each side grown nearer to 

 the mid line ; the notochord has thus thus become pinched into a thin vertical plate ; 

 the basioccipital and the exoccipital (Plate VIII. fig. 2, b.o.e.o.) also, have increased both 

 relatively and really, the cartilage within their laminae being much metamorphosed. 

 In the superoccipital region a large," irregularly hexagonal plate of bone has appeared, 

 reaching above to the converged parietals, and below already bounding the " foramen 

 magnum" (Plates VIII. fig. 4, s.o.). I have not been able to see this superoccipital 

 element on its first appearance in the " Struthionidae," but the only genus in which I 

 have found it as an originally azygous piece is Turdus (the Thrushes) ; even in Passer, 

 Erythacus, and Corvus — good representatives of three great, and eminently typical 

 families — there are two superoccipitals at first. And it may be also noted that this 

 element is, relatively, exorbitantly large in the Ostrich-tribe. 



The cartilage investing the auditory capsule has greatly increased in thickness, and 

 the prootic and opisthotic (Plate VIII. fig. 4, op.) bones have just appeared ; the latter 

 may be seen externally (its outer lamina) outside the base of the " posterior vertical 

 canal." 



The basitemporals have coalesced (Plate VIII. fig. 2, b.t.) and form a narrow band 

 reaching across beneath the skull from one internal carotid opening (i.e.) to the other ; 

 but they have also entirely coalesced with the terminal end of the true basisphenoid. 



