132 Part III. —Twenty-third Annual Report 
lines of greater opacity appearing white by reflected light, and these lines 
are arranged in bands separated from each other by bands of darker, 
more transparent bone. Each of these bands may indicate a year’s 
growth, being the summer growth separated from that of the next 
summer by a band where there is less calcareous matter. But it is 
difficult to decide exactly how many such annual zones are present. 
Even when one or two of the outer zones seein distinct, the number of 
the central earlier zones cannot be distinguished with certainty. One 
may count three at one time, and at the next attempt there seem to be 
four or five, and the total number always remains doubtful and uncertain. 
The otolith in cod and other Gadide is large and opaque, and by 
examining it as a whole nothing can be ascertained of its internal 
structure. It is therefore, according to Heincke, useless for the purpose 
of determination of the age of the fish. I have found, however, that the 
successive lamine of which it is composed can be seen quite distinctly 
in transverse slices simply cut from the central region of the otolith with 
a scalpel. Such slices are, of course, rather thick, and their surfaces are 
rough and irregular. Nevertheless, when they are placed in water in a 
watch-glass and examined with a low-power objective, they are sufliciently 
transparent to show the successive lamine of which the otolith is 
composed, and the laminz in certain zones being much more opaque than 
in the zones Letween these, the whole section is distinctly divided into 
regions which I believe to indicate the annual increments, and which, 
therefore, show the age of the fish. 
The otolith (¢.e. the sagitta or largest otolith) of the cod is somewhat 
elliptical in outline, with rather pointed ends, and two surfaces, one 
convex and rather smooth, the other concave and more irregular. The 
convex surface is turned inwards, ¢.e. towards the brain, and somewhat 
downwards, the concave outwards and upwards. The convex surface is 
marked by a shallow longitudinal groove, into which fits the ridge of 
sensory epithelium, called the macula acustica of the sacculus. The edge 
of the otolith is divided by radial grooves into lobes which are chiefly de- 
veloped on the concave surface, and the central part of the concave surface 
projects slightly as a convexity. Fig. 16, pl. viii., shows the appearance 
by transmitted light of a transverse slice as above described. ‘There is a 
central opaque nucleus surrounded by successive lamin which are thicker 
in the parts corresponding to the edges of the otolith than in those 
scrresponding to the surfaces. The nucleus is nearer to the convex 
surface than to the concave. ‘The nucleus is surrounded by a number of 
opaque lamine, and these are succeeded by a number of more transparent 
ones, Then comes another zone of opaque lamin, while the most external 
are again more transparent. According to my interpretation, the opaque 
zone represents the deposit of one summer, the transparent that of one 
winter, so that the two zones together represent the result of cne year’s 
growth and indicate one year of age. The fish from which the otolith 
figured was taken was therefore two years old. 
For practical purposes, to determine the age of a number of specimens 
quickly, I find the best method is to examine a few scales in water, 
noting the number of winter zones, and the age apparently indicated, 
and then to extract an otolith by splitting the skull in the median 
plane, and to cut a transverse slice of the otolith in the manner described 
above. In this way the conclusions drawn from the scales can be tested 
and confirmed or modified. It may be asked why I have not prepared 
thinner and more perfect transverse sections of the otolith by grinding 
down thick slices. I have tried this method in the plaice, and not found 
it very successful. The piece to be ground down, after one surface has 
been ground smooth, must be fixed on a glass slide with Canada balsam in 
