•528 PSOF. T. W. BRIDGE ON THE 



walls of the utriculi and those o£ the auditory caeca, the two 

 .structures being separated by the membranes which close the 

 auditory fontanelles ; neither is there any open tubular commu- 

 nication between the utricular and saccular portions of the 

 auditory organs of opposite sides of the head by loeans either of 

 sub- cerebral or supra-cerebral connections. In these features 

 Notopterus closely agrees with those Teleosts in which the first 

 method has been adopted. On the other hand, it is equally 

 evident that to some extent Notopterus approaches the second 

 type and resembles the Clupeidse in the fact that, for a part of 

 their course, the auditory caeca are enclosed in bony grooves and 

 for the terminal portion of their extent occupy the interior of 

 ibony culs-de-sac, and also in the oiigin of the caeca from an 

 anterior tubular portion of the air-bladder. 



It is perhaps worth remarking that some little variation iu 

 minor details exists amongst those Teleosts which offer examples 

 -of the first and second methods, more especially with regard to 

 the precise position of the auditory fontanelles and the nature of 

 the cranial or periotic bones which form their boundaries, and 

 also in the degree of tenuity of the closely related walls of the 

 auditory prolongations of the air-bladder. 



Por example, in Sjyarus {Box) the fontanelle is represented by 

 "Weber (13. tab. vii. fig. 62) as being situated posteriorly to the 

 foramen for the exit of the Vagus uerve. In PseudopTiycis 

 lacchus the auditory fontanelle is described by Parker {loc. cit.) 

 as situated between the basioccipital and the opisthotic, and 

 immediately beneath the Vagus foramen. In Holocentrwn 

 spiniferum, Gtlir., the fontanelle is bounded by the exoccipital, 

 opisthotic, and prootic bones, and is anterior to the Vagus fora- 

 men; and in Notopterus, as we have seen, the prootic, opisthotic, 

 and basioccipital encircle the fontanelle. Finally, in the Clupeidae 

 the fontanelle is a simple perforation in the prootic alone, and 

 hence is more anteriorly placed than in any other Teleosts. 

 There can be no doubt, I imagine, that the precise location of 

 the fontanelle and the nature of its limiting bones, are due to the 

 varying extent to which the auditory caeca are prolonged for- 

 ward, and perhaps also to variations in the size of the fontanelle 

 itself. 



Of much more importance is the degree of tenuity of the 

 walls of the auditory caeca at their point of contact or closest 

 relationship with the auditory organs ; for upon this will depend 



