518 PEOF. T. W. BRIDGE ON THE 



and the whole extent of the outer surface of its drum-head 

 membrane, while the outer wall of the sensory canal is in contact 

 with the external scaleless skin of this part of the head. 



Practically, therefore, the cavernous sensory canal and its 

 investing ossicles completely separate the cranial fontanelle and 

 its memhrane from the superficial skin. Anteriorly to the 

 fontanelle the sensory canal is still of considerable width, but, 

 as it passes dorsad to the orbital cavity, gradually contracts to 

 more normal dimensions. 



VI. The Aitditokt Organ. 



The utriculus and the sacculus of each side occupy a spacious 

 common recess excavated in the substance of the corresponding 

 lateral half of the large basioccipital (PI. 37. fig. 5, u.s.r.). So 

 large is the recess, that the outer portion of nearly the anterior 

 two-thirds of the bone is reduced to the condition of a thin 

 partially transparent shell, and corresponds to the externally 

 bullate portion to which reference has already been made ; while 

 a thin, vertical, bony partition in the axis of the basioccipital is all 

 that separates each recess from its fellow of the opposite side of 

 the skull. The two recesses are partially roofed in by the mesial 

 union of horizontal ingrowths from the two opisthotic bones {op.), 

 in a fashion which recalls the method by which the similarly- 

 situated saccular recesses are roofed by the exoccipitals in the 

 Siluroid Pishes, The utriculus occupies the anterior two-thirds 

 of the utriculo-saccular recess, and is relatively of large size. 

 The much smaller sacculus fills up the hinder third of the recess, 

 and is connected with the utriculus by a very short but obvious 

 ductus sacculo-utricularis. Anteriorly and dorsally, the utriculus 

 gives off a conical, forwardly-directed diverticulum, which occupies 

 a recess of corresponding shape excavated in the prootic *. Near 

 the origin of the diverticulum, the ampullary extremities of the 

 horizontal and the anterior vertical semicircular canals communi- 

 cate with the utriculus by opening into a small recessus utriculi. 

 The great utricular otolith, or " sagitta," is nearly as large as 

 the cavity of the utriculus itself ; and it is interesting to note 

 that, anteriorly and dorsally, the otolith is produced into a con- 

 spicuous, tapering, conical process, which extends into the 



* Cf. Eiclewood's account of the auditory organ in the Olupeida (9). 



