130 ME. W. K. PAEKER ON THE STEUCTUEE AND 



roof-scale of the Sturgeon which Professor Huxley, by a curious twin-mistake, has at 

 once compared to the " pterotic " of the Fish and to the squamosal of the air-breathing 

 Vertebrata (see 'Principles of Comp. Anat.' p. 205. fig. 82, F.). 



The outer anchylosed part of the opisthotic is oblong ; the inner free lamina is wedge- 

 shaped ; only a fissure separates it from the exoccipital, but a clear margin of cartilage 

 insulates it from the prootic (Plate X. fig. 7, op. pro. e.o.). This latter piece is at present 

 entirely within the skull (see fig. 2 of Plate IX., showing its absence on the outside), 

 its commencement being close to the internal meatus, as in birds generally, and not over 

 the ampulla of the anterior canal, as in Struthio (Plate VIII. fig. 10, pro.). In the 

 Sheep, when the foetal head is 2 inches 9 lines in length, the relatively small prootic 

 has commenced by two ossific patches at the same part, as in the African Ostrich. I 

 mention this to show how ornithically aberrant the Great Ostrich is in this respect. In 

 the osseous Fish the prootic, of necessity, is developed from an outer lamella, although 

 the bony matter creeps round into the skull behind the trigeminal nerve, and finds 

 all the good solid cartilage there is, especially towards the base ; it thus may be seen 

 skinning over the periotic basicranial bridge (see Huxley, op. cit. p. 167, fig. 68, 

 p.r.o.). 



Laterally the whole of the periotic cartilage is still soft in the ripe puUus of the Ehea 

 {supra), as is also the thick edge of the paroccipital wing (Plate IX. fig. 2, e.o.), which 

 is continuous with it, but within (Plate X. fig. 7, pro.) the prootic walls in much of 

 the cranium ; it is roughly hourglass-shaped, and has the multiperforate fossa for the 

 seventh nerve exactly in its centre. 



The epiotic (mastoid) has not yet made its appearance ; there is some room left for 

 it, however, in the cartilage which hides the junction of the anterior and posterior 

 canals, in that which lines the " lateral cerebellar fossa," and outside the crown of the 

 great " anterior semicircular canal " (Plate IX. fig. 2, & Plate X. fig. 7). 



The anterior sphenoid {o.s. p.s.) is as yet unossified ; its region is small, as in most 

 birds, and its alae (orbitosphenoids) are the mere outturned edges of its thickened top 

 (Plate IX. fig. 1, O.S.). The vertical part is rather thin below, and is bounded in front 

 by the interorbital fenestra, and behind by the common optic passage. A large fissure, 

 filled up by a remnant of the early membranous skull, separates the anterior sphenoid 

 from the great ala on each side; it is a temporary orbito-alisphenoidal fontanelle 

 (Plate X. fig. 8.), the merest trace of which can be seen just above the optic passage 

 in the adult. 



A large pear-shaped fenestra has been formed in front of the presphenoid (Plate IX. 

 fig. 2, i.o.s.) as large as that cartilage; the deficiency of cartilage here depends upon 

 the room required by the large eyes, which nearly touch each other, and by a growth of 

 the septum too rapid for the proliferation of the cartilage-cells. The band of cartilage 

 above this fenestra is about a line deep, most of it being ossified by the vertical ethmoid ; 

 in the adult it is nearly — in old age, perhaps, quite — filled in by periosteal layers of 

 bone. The vertical ethmoid (Plate X. fig. l,p.e.) is already a large plate of bone ; it 



