DEVELOPMENT OF THE SKULL IN THE CROCODILIA. 271 
through the walls (Pl. LXIII. fig. 4, h.s.c. p.s.c), and the anterior canal can be seen, 
partly, in the sectional view from the inside (fig. 8, a.s.c) in front of the “meatus 
internus ” (VIII). 
In this stage we can thus trace the original elements of the chondrocranium ; they 
are—(q) the basal plate (parachordals) and notochord, (6) the three prochordal bars, 
and (c) the olfactory and auditory sense-capsules; the eyeballs are not counted, because 
of their freedom from the rest of the skull, yet they affect its form very much by their 
bulk and shape. 
The skull is finished above by a huge membranous roof (fontanelle), for even in the 
occipital region there is, at present, no solid cartilage above. 
The ventral walls of the head are very contracted (Pl. LXII. figs. 3, 4), and only the 
first and second visceral arches are well developed, for the third has merely distal 
rudiments. 
The first arch or mandibular (Pl. LXIII. figs. 3, 4, Pl. LXIV. figs. 1-4, and Pl. LX VIII. 
figs. 1 & 9, g, mk, ar) is composed of an epi- and a ceratobranchial element; but these 
parts are very large. 
The first cleft (¢/'), seen in these dissections and sections, runs (already) in two 
directions, namely, obliquely inwards and outwards; inwards to form the rudiment of 
the complex system of Eustachian passages, and outwards to form the cavity of the 
drum—‘ cavum tympani.” 
At present the Eustachian opening is a mere lipped crescentic slit, with its concave 
border looking towards the postero-internal surface of the mandible (mm) ; its position 
in the throat is shown in the vertical section (Pl. LXIII. figs. 7, 8, c/'); and its actual 
form and extent in the subhorizontal sections (Pl. LXIV. fig. 4, and Pl. LX VIII. 
figs. 1-3, cl’). 
Below the mouth, in some of these sections (Pl. LXIV. figs. 1, 2, and Pl. LX VIII. 
figs. 7, 8, mk), the free mandible is seen to be a solid, somewhat flattened, rod in all 
its fore part; but behind (Pl. LX VIII. figs. 5, 6, 9, ar) it is considerably dilated to 
form the articular head and the angular process. On that process the main part of 
the next arch, the “ ceratohyal” (c.hy), rests, and not only rests, but is already fused 
with it, so that at this point the two arches are continuous. Below this conjunction 
(Pl. LXVIII. figs. 6 & 9, ar, c.hy) the angular process is seen to be short, and reflected 
downwards. 
The quadrate cartilage, or mandibular pier (P]. LX VIII. fig. 9, g), is very large, and 
its main part is crescentic, hooking in a faleate manner over the first cleft and the 
hyoid arch, along the fore part of the auditory capsule. 
The hinder, concave, bevelled edge is already forming the front boundary of the 
tympanic cavity; its lower end is the solid rounded condyle for the lower jaw (ar, mh) ; 
but its front margin is developed into a thin and somewhat bilobate process, this is the 
“ orbital process” so familiar to us in Chelonians and Birds ; it is the common rudiment 
Vou. XI.— Part 1x. No. 2.—October, 1883. 20 
