PROFESSOR OWEN ON THE EUSTACHIAN CANALS IN CROCODILES. 
525 
e and el, fig'. 5, Plate XLL, in the sing'ie Eustachian valviilai- outlet, .r, on the soft 
prominence beliincl tlie posterior nares, t/. 
The canals from the lateral orifices, el, are partially divided by a longitudinal ridge 
of bone projecting into them from their anterior wall : and the dilated lateral branches 
of the alisphenoid division of the median canal, es, are impressed by a longitudinal 
groove. I may also remark, that at the upper part of their place of confluence or 
termination, there is a median fossa leading to a small vascular canal. 
The tympanum of the Crocodiles, Plate XLII. fig. 10, is very extensive, by reason 
of the air-cells continued from it, not only into the mastoid, but across the basi- 
occipital and basisphenoid*, and into the exoccipital, supraoccipital-f-, alisphenoid 
and parietal bones 
By the dissection of a young Gavial of the Ganges, preserved in spirits, and a com 
parison of this with sections of the cranium of a full-grown specimen, I have satisfied 
myself that the third median system of Eustachian tubes, as well as the two lateral 
tubes, exist in the Gavials as in the Alligators and true Crocodiles ; only in the 
Gavial the common terminal canal of the median system is shorter, as is shown in 
Plate XLII. fig. 9, e, es, eo. 
It appears to have been still shorter in the extinct Teleosauri ; the posterior pri- 
mary division of the canal which penetrates the basioccipital forms, in the section of 
the skull of the Caen Teleosaur, a subcircular depression, which is filled with the 
matrix in the Parisian specimen. The anterior primary division, answering to es, fig. 9, 
plainly perforates the substance of the basisphenoid, as it ascends obliquely forwards, 
and therefore can by no means be regarded as the posterior termination of the nasal 
passages, which, in the Teleosauri, are surrounded exclusively by the pterygoids, as in 
all the existing forms of Crocodilia. 
With regard to the homologies of the above described complex Eustachian or 
palato-tympanic air-passages in the Crocodilia, the lateral bony canals, el' , fig. 7? ter- 
minating at the grooves, el, answer to the simple Eustachian tubes of lizards and 
mammals: the median canal, e, e', with its dichotomous divisions, is a speciality 
peculiar to the Crocodilia. 
I forbear, with my present limited experience of the living habits and actions of 
the Crocodilian Reptiles, to offer any hypothesis as to the function of the complex 
canals which conduct the air and would convey its sonorous vibrations from the nose 
to the ear: but one peculiarity I may suggest, as being probably related to the struc- 
tures in question, in which the Crocodiles and Gavials differ from all the Lizard-tribe, 
viz. that of habitually floating with the operculated meatus externus submerged, and 
only the eyes and the prominent nostril exposed above the surface of the water. Any 
noise in the air that might reach the floating reptile would, under such conditions, 
be conveyed to the tympanum by the canals conducting to that cavity from near the 
* On the Archetype of the Vertebrate Skeleton. 8vo. Van Voorst, fig. 9, p. 22, 1 and 5. 
t Ibid. 3. J Ibid. fig. 19, p. 120, 6 and 7. 
