356 REPTILIA. 



thicker, and terminates by a thick broad flat end in a hole that leads 

 into the vestibulum, and makes part of the sides of that cavity. The 

 length of this bone was about an inch and a half; but it must be shorter 

 in smaller turtles. This difference in size [according to age] is contrary 

 to [the state of the tympanic ossicles in] the human subject. 



The vestibular cavity in the bone is somewhat irregular, but not very 

 much so : it is a little flattened on its outer and inner surfaces. The 

 bony semicircular canals hardly deserve the name ; they are large holes 

 passing round a small piece of bone. Within this general cavity of bone 

 there is a lining of a spongy membranous consistence, which seems to 

 be divided into two ; one the outer, which is immediately compressed or 

 influenced by the tympanic bone ; the other, or innermost, next to the 

 skull, seems to be the communication of the semicircular canals, and to 

 go through a cellular network before entering the cavity. This cavity, 

 with the semicircular canals, is filled with a fine cellular substance 

 which is loaded with water. Through each of these bony canals passes 

 a small tube about the size of a hog's bristle : these [membranous semi- 

 circular canals] are attached to, or lost in, the partition between the 

 two cavities, and become much larger at this attachment, being there 

 somewhat bidbous [the ampullae]. They are of such a nature as to 

 keep circular, and I should suppose answer the purpose of the bony 

 semicircular canals in other animals : and the cellular membrane and 

 water surrounding them is to keep them suspended in their proper 

 places. I found a good deal of mucus in the second cavity like the 

 urine of birds 1 ; the inner surface is cartilaginous in the middle, and 

 membranous round the edges, so that it is moveable, and would seem 

 only a partition between this cavity and the [bony] semicircular canals, 

 but I found a small passage between it and the cavity of these canals. 



I could not find any openings into the cavity of the vestibulum ; but 

 one had an opening into the common cavity of the canals. 



There are two nerves ; the anterior is the largest : this nerve, as it 

 is going to pass out of the skull, divides into two, both of which pass 

 into the ear, and seem to be lost on the partition where the small canals 

 are lost. The posterior nerve divides also, and sends a small plexus to 

 the anterior part of the same partition. There is no fenestra rotunda, 

 only the opening [fenestra ovalis] where the long bone, answering to 

 the stapes, malleus and incus, terminates 2 . 



1 [The substance of a chalky nature, answering to the 'otolite' in fishes.] 



2 [Hunt. Preps. Phys. Series, Nos. 1578—1580.] 



