AS AX IXDEX OF AGE. 15 



different types, but notably developed in the Cyprinidae, for example, in which 

 the scales are penetrated by canals through which water may easily percolate. 

 We are indebted to Dr. Salbey for an interesting paper on the structure and 

 mode of growth of fish scales.* In this work Dr. Salbey commences with 

 some points on the structure of the skin, in which he reviews facts already 

 known, then he gives a brief description of the four types of scales (Placoid, 

 Ganoid, Ctenoid, and Cycloid) established by Agassiz, and lastly he deals with 

 the external characters, internal structure, and mode of growth in the Cycloid 

 and Ctentoid types. According to Salbey, there are two layers in scales, 

 (1) an external or superior layer and (2) an internal or inferior layer. The 

 superior layer develops at the expense of the superficial layer of the skin by 

 the deposition of calcareous salts at the interior of this layer. The inferior 

 layer is composed of superimposed lamella;, indefinite in number. These 

 lamella?, are not homogeneous but are of two kinds. They are arranged in 

 such a manner that a comparatively thin lamella is found ])etween every two 

 thicker lamelhie. These thin and thick lamellae difter in character. The 

 thick lamellae are colourless and calcareous in their nature, the thin lamellae 

 are yellowish and composed of a conjunctive substance, a kind of cement 

 (Kifffuhsfan:^. Thus the arrangement of lamellae is that of a conjunctive layer 

 disposed between every two calcareous layers. In making sections the con- 

 junctive layer resolves itself into fibrous elements, the individual elements of 

 which appear to follow tlie same direction. The number of superimposed 

 lamellaj has no definite relation to the age of the fish, as seen by a comparison 

 of the number of lamellae on fish of the same species, but of very diflerent ages. 

 He thinks, however, that as the lamellcB of older fish are thicker, and as the 

 dillerence in colour between lamellae does not appear, it is probable that con- 

 junctive lamellae calcify during progressing years and fuse with adjoining 

 calcareous lamellae. From this occurrence, it would result that the number of 

 lamellae, while really being greater in the older fish, does not appear to be so, 

 on account of the lines of separation between the old lamellae having dis- 

 appeared. On the preceding characters Dr. Salbey builds the following 

 interpretation of scale growth. In the membrane situated at the inferior \mvt 

 of the scale there takes place a periodic deposit of calcareous matter. This 

 membrane, impregnated with calcareous salts, represents the inferior lamella of 

 the scale. Between this calcified inferior lamella and the skin there appears 

 a new layer of conjunctive substance. After a varying lapse of time, this 

 new layer calcifies itself in its turn and so on. This mode of growth may 

 serve to explain, says the author, how it comes about tliat the inferior layer of 

 the scale is the largest, and why there is a softer layer present at the inferior 

 part of the scale. This softer layer is merely a layer of conjunctive substance, 

 which has been deposited upon the most inferior layer between that and the 

 skin. Besides these facts relating to the layers of scales, Salbey deals with the 

 concentric lines, the grooves, the focus, and corpuscles. 



Conceidric lines. The concentric lines have not any connection with the 

 lamellae which compose the scale, as one may easily show by vertical sections. 

 These lines or ridges only belong to the superficial layer, and thus one under- 



* Salbey, 1868. 



