DEVELOPMENT OF THE SKULL IN THE OSTEICH TEIBE. 153 



the hinder angles are premorse ; it is reptilian in its lateral angular extension, as in the 

 Emu. The growth of the exoccipitals [e.o.) in all directions has caused the vagus 

 and condyloid foramina (8, 9) to be enclosed in bone, the condyle to be protected by 

 bone laterally, and the opisthotics (outer laminae) to be no longer distinct from the outer 

 part of these occipital arches (Plate XIV. fig. 4, op.) ; they are at present, however, 

 mere islands of bone, being completely surrounded by cartilage, save at the opisthotic 

 or upper margin, externally. A distinct tract of cartilage separates the lateral from the 

 upper occipital (Plate XIV. fig. 2, s.o.) ; and the epiotic, the upper part of the opisthotic, 

 and the outer part of the exoccipital regions are still unossified. The large superocci- 

 pital comes nearer to the pentagon in shape than in the other ^' Struthionidse," the basal 

 margin is much broader, and the sinus-passages (s.c.) are further apart. At the mid line 

 this bone is feebly keeled ; on the inside (Plate XIV. fig. 5, s.o.) its vicariously epiotic 

 part is embracing the hinder half of the " anterior " semicircular canal [a.s.c.) ; whilst 

 for a line in extent this canal is unossified, the rest is covered in by the prootic [pro.), 

 but a large, vertically oblong tract of cartilage runs down from the top of the great 

 canal arch to the top of the inner face of the exoccipital. The opisthotic lies close to 

 this face in the inside (Plate XIV. fig. 5, op.), but is still separate, and a narrow strip 

 of cartilage intervenes between it and the prootic ; the epiotic has not made its appear- 

 ance, and the fossa, under the great arch, where it should come is much shallower than 

 in the other Ostriches (Plate XIV. fig. 5). The prootic is already large, its inner lamina 

 being the largest {pro.) ; the two bony plates are fused together over the ampulla of the 

 great anterior canal (Plate XIV. fig. 6, a.s.c.) (and indeed through the substance of the 

 cartilage), save at their margins. A considerable mass of cartilage still separates the 

 prootic from the basioccipital and basisphenoid below (Plate XIV. fig. 6) ; this is con- 

 tinuous with the remnant which intervenes between the two basal bones {b.o., b.s.). A 

 small patch of cartUage comes between the prootic and the alj sphenoid, and there is no 

 trace of the great " pterotic " crest of the fish (Plate XIV. figs. 5 & 6). A half-ossified 

 band of cartilage runs across the " foramen ovale" (5), making it double, as in the Carp. 

 If the semicircular canals were only walled in by a thin layer of bone, they would be seen 

 to be marvellously like those of the Lizard ; in both the Lizard and the bird these tubes 

 and their bulbs are a very beautiful structure. The angularity of the basioccipital, and 

 the two pairs of lateral outgrowths to the basisphenoid (Plate XIV. fig. 2, a.p., pr.p.), 

 mark the very mixed character of the skull-base of the young Cassowary, for the syn- 

 chondrosis is nearly as fully seen below as above ; more so than in any other bird, as far 

 as I know. Yet at this very place we see an important embryological and, as it were, 

 reptilian character ; for the broadish terminal end of the basisphenoid is not only bifid, 

 but the split is really an oval space (Plate XIV. fig. 2, b.s.), the remnant of the " poste- 

 rior basicranial fontanelle" of Rathke. My earliest specimens oi Struthio camelus do 

 not show this ; they only, like other birds, show the split above ; so also neither the 

 Emu nor the Rhea have this space below ; and I have described an Emu a week 

 younger than this Cassowary. 



MDCCCLXVI. z 



