166 PEOF. W. K. PAEKEE ON THE MORPHOLOGY 



This new growth of cartilage binds together the primary cornua trabecular between the 

 nasal sacs (ol, no) ; it is scooped both in front and behind. The scooping in front faces 

 the re-entering angle between the cornua, and the space there is often a real hole in the 

 Urodeles — a niesorhinal, vertical passage. The hinder scooping is a correlate of the 

 swollen fore brain which rests upon it, and from which the olfactory crura are given off, 

 right and left, to the nasal sacs (figs. 1, 2, c.tr, no). The internasal region of the cornua 

 is elevated, and on this rest the crescentic olfactory capsules, the simple rudiments of the 

 nasal labyrinth ; they are now beginning to coalesce with then trabecular undersetters. 



Between the eyes the trabecular have sent up a sharp wall to the brain-cavity, right 

 and left ; regionally, these walls answer to the orbitosphenoids and alisphenoids of the 

 higher Vertebrata ; and the optic and trigeminal nerves (2, 5) in their exit show the true 

 landmarks. But in this very primordial and ancient kind of skull the segmentation into 

 separate neural cartilages is quite suppressed ; only where the notochord lingers can any 

 trace of somatomic division be seen. 



This wall is steeper than it appears to be as seen from above and below (figs. 1, 2, tr) t 

 but a section (PI. XV. fig. 1, tr) shows its depth. Its inwards curve below makes a floor 

 to a slight extent ; and near both ends of the wall there is a slight attempt at roofing-in 

 of the skull (PL XIV. figs. 1, 2). The floor also is slightly chondrified backwards from 

 the internasal plate. In the young the front margin of this plate (in.c) is a crescentic 

 notch, finished by the cornua (c. tr). 



Behind, also, the floor is more finished ; for the investing mass ensheaths the notochord 

 near the foramen magnum, and the apices of the trabecular have coalesced in front of its 

 apex. The perfect arch of cartilage in the occipital region is, at the keystone (s.o), 

 about one third as broad as the floor ; but the floor is mainly finished by the enclosed 

 notochord. 



In front of that arch there is no cartilage in the roof, the thickest part of the inter- 

 nasal plate not rising more than halfway to the proper roof. 



The occipital condyles (oc.c) are well formed ; they are oval, looking inwards towards 

 the notochord, and being most seen on the lower side (fig. 2). The huge ear-sacs are 

 quite perfect as to cartilage ; the curves of the enclosed canals have thrust them into 

 the sides of the skull, aborting the alisphenoidal crest of the trabecular, so that those end 

 in front of the junction of the anterior with the posterior canal (fig. 1, a. sc, p.sc) in the 

 ear-sac. In front and behind the ampullar of the canals make the sac bulbous ; and the 

 horizontal canal (Ji.sc) forms the eave of the outer face, the rudiment of the tegmen 

 tympemi. The glossopharyngeal and vagus nerves (9, 10) emerge between the sac and 

 the occipital ring behind ; and the 5th and 7th (5, 7), or trigeminal and facial nerves, 

 escape between the sac and the cranial wall in front. 



In the fore part of the head the cornua trabecular are squarish ; their inner edge is 

 thickened whe.ie the simple crescentic nasal cartilage is becoming grafted upon them 

 (figs. 1, c.tr, ua). 



The outer margin of each cornu is cut away in a crescentic manner, and the nasal 

 sacs lie on and outside the bars ; the outer nostril is near the front laterally, but the 

 inner (J.n) is further back. Tbis inner nostril is bounded on its inner side by the vomerine 



