of the Fishery Board for Scotland. 



129 



compressed so as to form a flat disc. The thin layers on the two faces 

 being translucent, the surfaces of contact between successive layers are 

 seen as lines approximately parallel to the outer edge. The layers are 

 thin and very numerous, but they are grouped into broader zones by 

 differences of opacity. Each zone is usually distinctly defined from that 

 which succeeds it externally, the line of division being due to a sudden 

 increase in opacity in the layers which form the commencement of the 

 next zone. By transmitted light the more opaque layers appear dark, 

 and rhe more transparent layers light. All my figures show the appear- 

 ance of the otoliths by transmitted light, the otoliths being examined in 

 water as transparent objects. When the light is shut off from below and 

 the object seen by reflected light, the appearances are reversed, the opaque 

 regions appearing white and the transparent dark. My observations 

 agree closely with Heincke's description of the structure as seen by reflected 

 light, but I find that examination by transmitted light shows the structure 

 more distinctly. The central area or first zone shows minor subdivisions, 

 but the limits of these are not so distinct as the more external boundaries, 

 and the whole of this central area appears to be formed in the first year 

 of life. It consists of a central very opaque nucleus, followed sometimes 

 first by a transparent zone, then one more opaque, and then a broader 

 more transparent. But these minor zones are not always distinct, 

 while the limit of the whole central area is usually quite definite. 



The radiating lines are due to narrow grooves on the surface termina- 

 ting in notches on the edge, and seem to be formed by folds in the 

 membrane of the auditory vesicle containing the otolith. 



The scales of the plaice, like those of the cod, exhibit a number of 

 concentric lines formed by ridges on the outer surface of the scale, but 

 these ridges are very much finer, closer together, and less regular than 

 in the scales of Gadida?. In the anterior embedded portion of the scale 

 the ridges are divided up into short bars by radiating bands which appear 

 transparent by transmitted light, but in the posterior more superficial 

 part of the scale these radiating lines are absent, and the ridges appear as 

 continuous wavy lines. Separate sclerites cannot be distinguished as in 

 the cod, although doubtless the ridges and the radiating lines in the one 

 case correspond to those in the other. Successive zones can usually be 

 distinguished in consequence of the fact that the lines or ridges are closer 

 together in certain zones than in others. A complete zone may be con- 

 sidered to be the result of one year's growth. The summer's growth com- 

 mences with lines or ridges which are rather far apart, and after a certain 

 distance the lines become more closely crowded ; then the next summer's 

 growth is indicated again by lines farther apart (fig. 10, pi. viii.) The 

 transition from the crowded lines to those further apart on the outside is 

 somewhat sudden, so that the commencement of the new summer's 

 growth is often fairly distinct. But in most cases the zones are somewhat 

 difficult to distinguish, and it would be by no means easy to form a con- 

 fident judgment of the age of the fish by examination of the scales alone. 

 The conclusion drawn from the scales must be confirmed or tested by 

 examination of the otolith. 



I have not found any sufficiently distinct lines of growth in the oper- 

 cular bones, as Heincke states, but such lines are visible in the elements 

 of the pectoral girdle and in the concave faces of the vertebra?. In neither 

 case, however, are such good indications given as in the otolith. I have 

 represented the appearance of a vertebra and of the pectoral elements in figs. 

 13, 14, pi. viii. The pectoral girdle of the plaice consists of a somewhat thick 

 ossified cleithrum, thicker and less expanded ventrally than that of the cod, 

 and a scapula and coracoid consisting chiefly of calcified cartilage. The 

 coracoid comprises two parts, a thin ossified ventral portion bordered by a 



