DEVELOPMENT OF THE SKTJLL IN THE CEOCODILIA. 279 



now more than twice as long as the parachordal tract. The whole base gradually 

 lessens forwards, but at its middle, in the ethmoidal region, the lateral elements (tra- 

 beculse, tr) cease. The intertrabecula then enlarges somewhat, gradually lessens again, 

 and ends in front of its own crest, the septum nasi, in a spearpoint-like process, the 

 prenasal rostrum (p.n), which turns a little downwards, and projects slightly beyond 

 the nasal labyrinth. 



The flattened trabecule, after giving off the orbito-sphenoids (o.s), swell into a short 

 solid wedge ; the two wedges fill in the space between the hind lobes of the nasal 

 capsule below: they are the cornua trabeculse (c.tr). These cornua in short-faced 

 forms, such as the Amphibia, spread into the fore face ; here, in the race of growth, 

 they are left midway by the fast-growing intertrabecular bar. 



The trabeculee almost touch each other above, where they give off the orbito- 

 sphenoidal wings (fig. 1, o.s) ; these together form a lozenge-shaped hammock for the 

 fore brain to lie on, and, whilst ending in a sharp point on each side in front, are tied 

 by strong cartilaginous tapes to the posterior wings, or alisphenoids (al.s). 



The large pyriform anterior fenestrse, and the small oblong fenestrae behind them, 

 bring this skull very near that of the Lizard ("Skull of Lacertilia," Phil. Trans. 1879, 

 pi. 4-3). The ethmo-nasal crest of the middle bar (j^-e) is exposed in its hinder part, in 

 front of and between the orbito-sphenoids ; it there forms a sort of " crista galli" between 

 the olfactory lobes, that rest upon a concave part of the roof of the nasal capsules. 

 This hinder, more swollen part of these capsules corresponds to the ethmoidal region in 

 the Mammal, but is very simple within. After a sudden contraction the nasal roof and 

 walls are enlarged again, sinuously, but, on the whole, are gently lessened up to their 

 fore end. The olfactoiy nerves pass through a single hole on each side behind the 

 higher part of the roof in the front of the rhinencephalic recess. The walls in the true 

 nasal, as well as in the ethmoidal, region, pass across, below, and form a very complete 

 floor, becoming confluent (as I shall show in the next stages) with the intertrabeculae 

 for some extent. 



In front, they are bulbous below, and the semi-distinct alse nasi {al.n) form two upper 

 bulbs twice as large as those below ; these latter are perforated in their middle (above). 

 This crescentic passage, with its horns looking backwards, is the external nostril (e.n). 

 Here the connate, circular, valvular fold is so specialized as to open on the %ipper 

 surface of the head, whereas the primary position of the opening is below. 



The auditory capsules [cm) are only confluent with the rest of the chondrocranium 

 below; they are very large, and have lost much of their original ovoidal shape. The 

 semicircular canals (a.s.c, Ii.s.c, p.s.c) are large and shine through the unossified 

 cartilage; so also do the other processes of the membranous labyrinth. A large 

 pyriform tract of each capsule is seen on the upper surface of the skull ; its narrow 

 end is in front, and that part projects outwards, and is produced into an angle. 



VOL. XI. — PART IX. No. 3. — Octoher, 1883. 2 x 



