REPORT ON THE GREEN TURTLE. 9 



mandible (mn.), already assuming the veiy image of the quadrate, with its tympanic 

 cavity and its condjde. 



Then, iu the hollow of the crescent, a discoid body has developed ; this is caused by 

 the distal part of the upper segment of the hyoid arch — the circular extrastapedial ; 

 the rest of the arch is forming in the second post-oral fold (%.). Between this elevation 

 and the hollow of the quadrate elevation, we see the beginning of the membrana tympani. 



Behind the discoidal elevation two lesser prominences are seen ; these are caused by 

 the thick post- tympanic ligament, and the digastric muscle. 



The fold of the angle of the mouth {mx.p.,m.) still shows itself to be as true an 

 " opercular fold " as those which succeed it ; it is the post-lachrymal part of the huge 

 maxillo-palatine fold, as the thick ethmoidal end is the pre-lachrymal part ; that cleft 

 lies between these two regions (see PI. II. figs. 2, 3, l.cl.,mx.p.). 



The lateral view {PL I. fig. 7), the directly inferior view (PL II. fig. 2), and the view 

 from below, but with the head tilted back, and with the lower arches removed (PL II. 

 fig. 3) — all these must be compared together to get a clear conception of the parts round 

 the mouth. 



The extent to which the supra-oral parts are modified by the eyeball is evidently 

 great ; that organ is now at its relatively largest size, exceeding the mid-brain in bulk. 



This preoccupation of so much of the face, and the enfolded form of the whole 

 head, have wrought great changes in the shape of the series of the facial folds. The 

 evidence in favour of the view of the double nature of the great sub-ocular fold 

 (" maxillo-palatine," mx.p.) is as follows :— 



In this stage it is somewhat hour-glass shaped ; it develops a head-cavity in its hind 

 part, and not in its fore part ; the lachrymal cleft is over its narrow part (PL II. fig. 3, 

 l.cl.,mx.p.) ; and in many kinds of Vertebrates there is a distinct visceral cartilage in the 

 fore part, and another in its hind part — one the true endoskeletal-palatine or " ethmo- 

 palatine," and the other the " epipterygoid." 



The space between the maxillo-palatine fold and the " fronto-nasal process " (foremost 

 right and left visceral folds in one lobe) is quite distinct and clear ; and the outer 

 part of the nasal roof articulates with the maxillo-palatine fold. This outer limb of 

 the nasal paraneural cartilage contains the rudiment of the "lateral ethmoid," which, 

 however, becomes complicated, in many types, by fusion with the trabecula and the 

 ethmo-palatine cartilage, or axis of the fore part of the maxillo-palatine fold. 



The large lachrymal cleft lies over this junction, but indeed organically close behind 

 it. Yet both the lateral and inferior views suggest the direct serial homology of the 

 fronto-nasal with the maxillo-palatine in its fore part, and the chink between them may 

 be called a cleft, whether its homology with the post-oral clefts be conceded or not. 



The double terminal part, the fronto-nasal pnvcess {nfp.) contams both the 

 inner Hmb of the crescentic nasal roof-cartilage, and also the foremost part of the basi- 



(ZOOL. CHALL. EXP. — PART V. — 1880.) ^ 2 



