AS AN INDEX OF AGE. 19 



scales are isolated and completely enclosed in the skin (Anguilla, Ophidium 

 harhatum, etc.) the long axis of the scale does not usually show any fixed 

 jiosition as regards its direction. 



It appears probable that the reciprocal pressure exercised by scales the one 

 upon the other, contributes so far in bringing about a similar mode of orienta- 

 tion anion" them. 



3. The she of scales. The size of scales is extremely variable. They even 

 show varying dimensions on different parts of the body of the same fish. For 

 instance, the scales on the flanks are larger than those near the caudal fins. 

 Scales gradually diminish in size from the median region of the side towards 

 the tail or anus. Scales are also reduced in size in various parts of the head, 

 in the opercular and preopercular regions and in the suborbital region. 

 Baudelot gives tables sliowing the variation in size in different regions of the 

 body for the perch, pike, and mullet. These tables show in what proportions 

 the size of scales varies with the age and size of the fish mentioned. Growth is 

 continuous but unequal in scales from different regions of the body. One finds 

 very great variation in the size of scales in different species of the same family 

 of fish, and certain varieties of the same species show extreme differences in 

 the relative sizes of their scales. For example, the so-called mirror carp has 

 very much larger scales than those of the ordinary carp. In another variety, 

 the leather carp, the scales have become very rudimentary or have entirely 

 disappeared. 



4. The ridges on scales (cretes de I'ecaille). In cycloid and ctenoid fishes the 

 surfaces of the scales show linear projections which are usually parallel to the 

 external contour of the scale. Baudelot describes the arrangement of these 

 under the term " cretes de recaille." Though these ridges are almost con- 

 stantly present, yet in several types of fishes they disappear more or less 

 completely, for example, in Dadijlopterus volitans and the tunny. In the 

 tunny one finds some scales provided with as many ridges as usual, others 

 with ridges only at the margins, and others in which these ridges are com- 

 pletely wanting. In the eel, scales do not show ordinary ridges on their 

 surfaces ; these are replaced by reliefs of a quite distinct appearance, but 

 really of the same nature as the ridges. 



In regard to the disposition of ridges on the scale surface there is consider- 

 able variation in different fishes. In certain types of scales, those of the 

 salmon for exami)le, the ridges run parallel to the contour of the scale in a 

 perfectly regular manner, thus forming a series of continuous reliefs which may 

 truly be termed " concentric ridges." In other types of scales, those of the 

 pike, some Cyprinidse and Pleuronectidaj, for example, the concentric ridges 

 show some degree of regularity in the peripheral portion of the scale, but as 

 they approach the centre of growth they lose their uniformity, become inter- 

 rupted at various points, bent in various directions, intersected by secondary 

 ridges, and finally appear like a " veritable labyrintli." In regard to this 

 point there are the greatest differences between scales of the same fish. 



In many scales the characters of the concentric ridges undergo a greater or 

 less change in the posterior region of the field ; sometimes they may entirely 

 disappear (herring, shad), sometimes they become very rare, they separate the 



