MOEPHOLOGY OF OPISTHOCOMUS CEISTATUS. 49 



Stage A. 2f inches long ; about half ripe. 

 Stage B. 3^ inches long ; about two-thirds ripe. 

 Stage C. 4J inches long ; about three-fourths ripe. 



The largest embryo was evidently not quite ripe for hatching, and the smallest was 

 at any rate half ripe ; I come to this conclusion by comparison of these embryos with 

 those of the Common Fowl. I am in a position now to compare the developing 

 skeleton of this rare type with that of its familiar relative, for the descriptions and 

 figures of the skull, shoulder-girdle, and wings of the latter are already published 

 (see Phil. Trans. 1869, pis. 81-87, Proc. Eoy. Soc. 1868, and Phil. Trans. 1888, B, 

 pi. 62) ; and the rest of the skeleton of the chick has been worked out by me and. will 

 soon be published. 



III. The Skull of Opisthocomus cristatus in the Emhryos and Adult. 



The chondrocranium in the first stage is at its fullest development, and is beginning 

 to undergo ossification in certain parts (Pis. VII., VIII.) ; it is now in the best state for 

 comparison with the cartilaginous, or osseo-cartilaginous, skull of the Ichthyopsida, and 

 with the early condition of the skull in Reptiles and Mammals. The solid fore part of 

 the premaxillaries ipx.), where the right and left bony tracts are already fused together, 

 is as long as the vestibular region of the nasal labyrinth (PI. VII.). In this short- 

 headed, bird the brain-cavity extends only along the hinder two fifths of the whole 

 chondrocranium ; but its axial is only part of its real length, as it is tilted upwards 

 very much, whilst the auditory capsules are tilted downwards and backwards so as to be 

 almost horizontal (see 3rd stage, PI. VIII. fig. 2). The whole structure, when 

 deprived of the investing bones, is a short and rather shallow basin behind, with a long 

 high wall in front. The foremost part of the face in front of the wall, and one-fourth 

 of its length, is the free, rounded, somewhat flat, prenasal rostrum of the inter- 

 trabecula ^. 



The nasal labyrinth is like that of the Common Fowl, but the inferior turbinals 

 (PI. VIII. fig. 4, i.th.) are simpler ; they make scarcely more than one turn of a coil. 

 In the normal ornithic skull we have repeated the high type of cranium seen in so 

 many osseous fishes ; whilst in Mammals the low type of cranium, seen in the Skate 

 and the Frog, is once more adopted. This is the more important to notice because of 

 the fact that of all the Iclithyopsida the high-skulled Teleostei are the most instructive 

 types with which we can compare the Carinatse, with their marvellously elastic and 

 mobile jaws and palate. In these high skulls the posterior sphenoid, only, is fully 



^ In the Apterys on the one hand, and in the Ibis on the other, the notochordal region of the skuU is extremely 

 short as compared with the prochordal ; in Opistlioeomus the latter, in this stage, is more than half as long as 

 the notochordal, even allowing for the upward tilting of the notochord between the moieties of the post- 

 pituitary or clinoid walls. 



