Rate of Growth in Gubius minutus. 235 



toli (lis. 



The otoliths can first be observed in specimens of G. minu- 

 tus upwards of 4 mm. An otolith extracted from a 12 mm. 

 form is figured in PI. X. fig. 8, as seen by reflected light. 

 At this stage the form is plate-like, and the concentric lajers 

 are visible under a low power of the microscope. In a form 

 of 19 mm., mounted in balsam, the otolith can be seen very 

 clearly ; it is on a line with a point rather above the centre 

 of the eye, and about a diameter of the eye from the outer 

 edge of the latter. PI. X. fig. 9 represents an otolith from 

 a 33 mm. fish as seen by reflected light. The central portion 

 is not very thick ; then follows the denser growth of the 

 summer and early autumn, and finally the thin growth of 

 the late autumn. 



From this point onwards the otoliths of G. minutus are 

 concavo-convex in shape, and are so opaque that it is quite 

 impossible in most cases to make out the internal structure. 

 The otolith lies in the auditory capsule, placed slightly poste- 

 rior to the eye ; the convex surface is turned outwards. As 

 lias been stated before, when the fish had been preserved in 

 formaline, the otoliths are either entiiely absent or are merely 

 represented by a mass of pulp. Difficulty was experienced in 

 devising a method for observing the structure of the larger 

 otoliths. Sectioning was tried with a scalpel, but in most 

 cases only resulted in splitting the stone into fragments. 

 Finally, a clean otolith was placed upon a glass slide and 

 covered with a drop of melted wax. When the latter had 

 hardened, the wax covering the convex surface of the otolith 

 was carefully removed. A drop of dilute acid (HC1) was then 

 applied to the exposed surface. By watching the action of 

 the acid on the calcium carbonate under a low power of the 

 microscope it was possible to check the process (with excess of 

 water) when the otolith was sufficiently thin. The wax pre- 

 vented the acid from attacking the sides of the otolith. The 

 wax was removed by heating the slide gently over a Bunsen 

 flame, and then washing with a few drops of xylol. During 

 this process if the otolith had been worn down excessively it 

 not infrequently broke up. It was then mounted in Farrant's 

 Solution. This method of observing otolith-structure is not 

 of much practical value, for much time is required for each 

 observation and so many failures occur. It may further be 

 questioned whether much reliance can be placed in results 

 obtained in this way, for many of the markings may have 

 been induced by the action of the acid. However, a series of 



17* 



