[mcuurricx] LIFE CYCLES OF THE PACIFIC COAST SALMON 15 
The otoliths are composed of an oiganie matrix infiltrated with 
calcium carbonate, so that when the latter is removed by hydrochloric 
acid a perfect organic “skeleton” remains. The growth in size takes 
place by the addition of concentric rings, and since the relative amounts of 
the organic and inorganic constituents vary at different times, thereby pro- 
ducing differences in the translucence of the material deposited, growth 
periodsare more or less clearly marked out. When the organic constituent 
is relatively more abundant in a portion of a ring, that portion appears 
darker by transmitted light, and, contrariwise, the portion in which 
the inorganic material is more abundant appears clearer. These ap- 
pearances are reversed when the otolith is viewed by reflected light, but, 
following Fryd’s example, the terms lighter and darker will be applied 
as if the structure has been examined by transmitted light, since this 
was the way in which they were first applied by Reïbisch.! According 
to this latter author the occurrence of the larger amount of organic 
material denotes a higher temperature, so that the clearer portion of 
each ring is supposed to represent winter growth. 

ae 
Sonn à 

“ es, 
Aria, DA TRS as 
Su, à esd 
Fig. 1. Fig. 2. 
Text-fig. 1.—Left Sagitta of Sockeye Salmon, 68 cm., in length, seen from the lateral 
surface. 
Text-fig. 2.—Right Sagitta of Sockeye Salmon, 68 cm., in length, seen from the 
medial surface. 
In shape the sagitta of O. nerka (Text-figs. 1-3) justifies the term 
applied to it, being distinctly arrow shaped. The posterior border 
always presents a distinct notch and one is usually to be seen at the an- 
terior tip of the ossicle, at the extremity of what Fryd has termed the 
rostrum. The antirostrum is very poorly developed, the excisure being 
either hardly noticeable or indicated by a slight depression on the upper 
border of the otolith. The sulcus (Text-fig. 2) is complete, extending 
throughout the entire length of the ossicle, although at about its middle 
it is usually interrupted by the fusion of its dorsal and ventral lips. 
In adult fish the ossicle has a greatest length of 4.75—5.0 mm., 
and a greatest breadth of 3.0—3.5 mm. Towardsthe centre (Text-fig. 

‘Loe cit. 
