LOCATION OF THE SACCULUS IN FISHES. 105 



Pacuma of British Guiana). Differing, then, as they do in the 

 quantity of bone with which the sacculus is surrounded, there is 

 also a difference as to the roominess of the bony pockets. With 

 the Ostariophysi the sacculus fits compactly in the cavity 

 provided for it, with no room for movement. In Brama rail 

 (Ray's Bream, Couch), on the contrary, the cavity is much longer 

 than the sagitta in its sacculus, and this could be moved 

 backwards and forwards freely. In the Sciaindce also the 

 pocket for the sacculus is roomy. In those fishes having a well- 

 defined lapillus, the third otolith, there is always a cavity in the 

 skull for the " Recessus utriculi " to rest in, this being that 

 part of the ear labyrinth holding the lapillus. Notably is this 

 so in the Siluridce, to accommodate what in some of this family 

 is a very large stone ; this cavity is well-defined on the outside 

 by a bulging-out of the bone of the skull : this is clearly seen in 

 Arius spixii. 



To get a more definite idea of the location of the sacculus 

 and of the otoliths it will be as well to take the families seriatim 

 of such as have been available for examination. 



POLYPTERID^. 



Polypterus senegalus has the pocket for the sacculus high up 

 at the side of the skull and open ; the sacculus could be extracted 

 with a little manipulation without cutting away any bone. In 

 this fish the asteriscus* is the largest of the three otoliths. 



Acipenserid^e. 

 Acipenser sturio (the Sturgeon). In this fish the sacculus 

 contains otoconie as well as a solid sagitta. 



Amiidje. 

 Amia calva (the Bowfin of North America) has the asteriscus 

 as the largest of the three otoliths on each side. The location 

 of the sacculus and its contents is shown from the outside by a 

 swelling on the basi-occiput of thin shiny bone. The sacculus 

 and labyrinth of the ear are situated in a recess separated from 

 the brain in the cranial cavity by a diaphragm of cartilage. 

 The sacculus is in two well-defined portions, the lagena, the 

 posterior and larger one, holding the asteriscus (fig. 8, p. 109). 



* Figured in the ' Zoologist ' (vol. xiv., p. 294, pi. ii. f. 5). 



