DEVELOPMENT OF THE SKULL IN THE CROCODILIA. 287 



notched behind, and has two rounded notches antero-externallj' ; there is but little 

 evidence of a hypohyal rudiment, except the dilated sides of the plate in front of 

 the " first ceratobranchials " or thyrohyals (c.ir') ; these latter are inbent rods, largely 

 ossified. 



c. Investing Bones. 



These were worked out in the largest embryos of the Alligator (Pis. LXVI., LXVII.) ; 

 they are already very characteristic of the type, and are much in advance of what has 

 been described in the third stage. The main fontanelle (PI. LXVI. figs. 1 & 4, yb) is 

 very large as yet ; the parietals [p), only flank its sides as small ear-shaped plates ; they 

 have a projecting angle inwards, and look like squamosals. The frontals {f) form a 

 sort of " beading " round the large orbits ; the upper part is a narrow, crescentic band, 

 gently widening from behind forwards. The largest part is the concave orbital flange, 

 turned inwards along the whole length of the bone. The frontals overlap the parietals 

 behind, and are overlapped by the prefrontals in front. 



The latter bones {p.f) are convex and ear-shaped, with a scooped hind face against 

 the eyeball in front ; the two are their own width apart on the top of the head. 



In front of these are the nasals (h) ; they are almost oblong, but are narrow in front ; 

 they have a concave fore margin obliquely fitted to the alse nasi (al.n), and are pointed 

 there at their inner edge. 



Tlie premaxillaries (^w) form together a semicircle, broken in the middle, where 

 there is a gap between them, showing the prenasal cartilage {p-n); these and the 

 maxillaries have a double wall, a deep common alveolar groove, and fast-growing 

 teeth in it. 



On each side of the alveolar groove the maxillaries {mx) are well developed both 

 externally and within ; in the former region there is a large facial plate running from 

 the premaxillary in front to a line below the optic nerve (ii) behind ; this suborbital 

 part is narrow, scooped above, and pointed at the end. The palatine edge of the 

 premaxillary is small, that of the maxillary is a large ingrowing plate, widest in the 

 middle, but half its own width from its fellow. Between the maxillary and the 

 prefrontal there is a small triangular bone applied to the lacrymal involution ; this is 

 the lacrymal bone {I). Close behind it there is a styloid bone, curved upwards in 

 front, downwards behind, and having at its hinder third an ascending triangular 

 process; this is the jugal {j). A similar process comes down from a bone above, 

 finishing the postorbital rim ; this is from the postorbital bone (p.oli), the upper part 

 of which is a crescentic shell with an outer and an inner toothed process. The relation 

 of the postorbital to the parietal is antero-external ; it clamps the frontal, parietal, 

 jugal, and squamosal. The latter bone {sq) is a large convex trowel, with its " handle," 

 in front, overlapped by the postorbital. 



These two bones are separated from the skull-wall by a deep chink — the temporal 

 space ; but, behind, the squamosal strongly clamps the auditory capsule, and by its 

 VOL. XI. — PAKT IX. No. 4. — October, 1883. 2 y 



