DEVELOPMENT OF THE SKULL IN THE CEOCODILIA. 271 



through the walls (PI. 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 — (a) the basal plate ( j;arachordals) and notochord, (b) the three prochordal bars, 

 and (f) 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 (PI. 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 (PI. LXIII. figs. -3, 4, PI. LXIV. figs. 1-4, and PI. LXVIII. 

 figs. 1 & 9, 5', mk, ar) is composed of an ejji- and a ceratobranchial element ; but these 

 parts are very large. 



The first cleft {cl^), 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 {mn) ; its position 

 in the throat is shown in the vertical section (PI. LXIII. figs. 7, 8, cl^) ; and its actual 

 form and extent in the subhorizontal sections (PI. LXIV. fig. 4, and PI. LXVIII. 



figs. 1-3, cr). 



Below the mouth, in some of these sections (PI. LXIV. figs. 1, 2, and PI, LXVIII. 

 figs. 7, 8, mk), the free mandible is seen to be a solid, somewhat flattened, rod in all 

 its fore part ; but behind (PI. LXVIII. 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 

 (PI. LXVIII. figs. 6 & 9, ar, c.hy) the angular process is seen to be short, and reflected 

 downwards. 



The quadrate cartilage, or mandibular pier (PI. LXVIII. flg. 9, q), is very large, and 

 its main part is crescentic, hooking in a falcate 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, mk) ; 

 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 



VOL. XI. — PART IX. No. 2. — October, 1883. 2 u 



