424
PROF. R. OWEN ON THE MODIFYING INFLUNCE
nares/ in the existing Crocodiles and Alligators of Asia, Africa, and America, the power of holding submerged a powerful mammiferous quadruped, without permitting the streams of water traversing the great cavity of the mouth during the struggle to get access to the posterior nostrils and wind-pipe of the amphibious assailant.
The valvular mechanism applicable to, or, I may say, possible with, the peculiar position of the posterior nostrils of procoelian Crocodiles, opening vertically behind the bony palate, not hori- zontally upon that plane, could hardly be adjusted to the relatively larger postpalatine apertures, upon a horizontal plane at some distance from the occiput, with the inner nostril opening at a more advanced position in the mouth — an arrangement which charac- terizes all amphicoelians.
Pigs. 4-6. — Cranium of Teleosaurus.
4
v ""^lalB^^*
wrr- "y ' —
^'wjD^^K
4. Lateral view.
5. From above.
6. From below.
Explanation of letters as under figs. 1-3.
No doubt there were sphincteric structures which would exclude water from the glottis in all the aquatic air-breathing reptiles; but the peculiar and well- developed valvular contrivances to that end in existing crocodiles are conditions of the relative size and position of the posterior nostrils in them, and the repetition of that character in the palato-nares of all known Tertiary crocodiles justifies an in-
