136 ME. W. K. PABKEE ON THE STEUCTtJEE AND 



a broad isthmus ; within it is free (Plate XI. fig. 7, op), as in the Crocodile and the 

 Rhea. The condyloid foramina, and those for the vagus (8, 9), are quite surrounded by 

 the bony matter of the exoccipitals ; the paroccipital ala is still soft (e.o.) at its thick 

 margin. The large lozenge-shaped superoccipital (s.o.) forms nearly a right angle by 

 the meeting anteriorly of its parietal margins ; these edges are not straight, but convex 

 at the ends and concave at the middle. More than a fourth of the occipital foramen is 

 formed above by this bone (Plate XI. fig. 4, s.o.). The great sinus-passage opens out 

 4 lines from its fellow in a deep groove, beyond which the bone is fast extending (Plate 

 XI. fig. 2, s.c.) ; the whole breadth of this (once double) bone is 6^ lines, its height 

 6 lines. The basisphenoid is largely ossified, and the small basitemporals, pushed from 

 each other by the azygous piece, having coalesced with its sides behind (Plate XI. 

 figs. 2 & 7, b.s.b.t.), they form the hindermost of three pairs of wings, now gi'owing 

 out of the thick part of the great basisphenoid. These lingular elements are 2 lines 

 long by 1^ line in breadth; they are grooved on their upper surface by the internal 

 carotid (i.c.). The middle pair of wings grow directly out of the body of the basi- 

 sphenoid ; they are the thick, broad " posterior pterygoid processes" (jor.p.) ; they are 

 deeply scooped from behind forwards to give capacity to the ear-drum. Between these 

 two pairs of wings there is a shallow groove, the badly defined "Eustachian tube" (Plate 

 XI. fig. 7, Eu.); and between the posterior and anterior pterygoid processes (a.j).) 

 there is a notch, a groove, and a foramen. The latter processes (anterior pterygoid) have 

 their usual large size, and are turned forwards as well as outwards ; between these spurs 

 there are a few vascular passages where the membranous pituitary space once existed. 

 The rostrum (r.b.s.) fails of its gigantic size in this genus, but at this early stage it is 

 I inch long and a line in thickness at its root ; it reaches nearly to the alse nasi. The 

 deep prepituitary portion of the basisphenoid is already far advanced ; and the alisphe- 

 noids are almost completely ossified (Plate XL fig. 1, b..s.a.s.) ; within, a nearly hori- 

 zontal ridge divides the upper third from the lower two-thirds, both of these spaces 

 being concave ; the high postero-superior angle entirely shuts out the squamosal from the 

 cranial cavity; the postfrontal process is still soft (Plate XI. fig. l,pf.). We saw that 

 the outer lamina of the opisthotic had coalesced below with the upper edge of the ex- 

 occipital ; within, the fan-shaped inner plate has all but coalesced with the exoccipital, 

 whilst a clear band of cartilage divides it from the prootic. This reptilian disposal of 

 the " otic " centres prevails throughout the Bird-class, and is in perfect harmony vsdth 

 numberless other erpetic characters. A large space of cartilage separates the prootic 

 above from the epiotic process of the superoccipital (Plate XI. fig. 4) ; in this the small 

 epiotic would have appeared, as we shall see in our next example. The " pterotic " 

 region is small and unossified ; it will be divided equally between the already large 

 prootic and the alisphenoid, but the bony centre itself has not yet shown itself to me in 

 the Bird-class. The prootic (Plate XI. fig. 7, pro.) is a very internal bone, as in the 

 Rhea ; although large it is completely insulated by unchanged cartilage ; half the arch 

 of the anterior, and the posterior half of the horizontal canal are still imbedded in car- 



