AS AN INDEX OF AGE. 21 



concentric ridge. Tranveree sections, that is to say, sections perpendicular to 

 the surface of the scale, also show this point. The separation of the ridges 

 is not great, and does not appear to vary witli age ; the latter point evidently 

 proves that the scale does not grow at all points on its surface. The distance 

 separating the ridges from one another may remain the same in the different 

 regions of the scale ; but this is by no means constantly the case. In the 

 sole, for example, the ridges are much closer in the anterior than in the 

 lateral part of the scale ; and in most cycloid scales the ridges of the posterior 

 region show a greater degree of separation than those of the lateral and 

 anterior regions (minnow, Cyprinus, etc.). 



^^ It is also not uncommo7i to find in the same area of the scale successive 

 zones in the extent of tchich the ridges show different degrees of separation." 

 (See figures of carp scales.) 



From the following facts, Baudelot concludes that the ridges do not 

 represent by any means the borders of superimposed plates or lamellae, as 

 many zoologists had supposed ; but that these ridges, whether they be con- 

 centric lines or not, are nothing else than reliefs corresponding to lines of 

 calcification at the external layer of the scale. 



(1) The ridges only very rarely affect a complete arrangement in the form of 

 concentric lines. 



(2) These ridges may be perpendicular to the contour of the scale. 



(.3) These ridges may show the most irregular arrangement, become folded up 

 against one another, entangled in all directions, or even form a sort of network 

 of irregular meshes. 



(4) The ridges are appendages to the superficial layer of the scale, 



(5) They originate at the margin of the scale as points of isolated calci- 

 fication. 



(6) They show a marked inclination towards the centre of the scale. 



5. The spines (spinula). Under this heading Baudelot discusses the small 

 spinous projections seen in the posterior portion of ctenoid scales. The 

 variation in the form of those appendages is very great, affording transitions 

 from simple denticulations to true teeth. In the tunny, for example, we find 

 quite simple denticulations or cuttings in the posterior border of the scale 

 which cannot be regarded as distinct organs, but simply as projecting lobes of 

 the free border of the scale. At a further stage (some species of Sargus) 

 denticulations prcjject from the concentric ridges of the posterior field. This 

 is really only a more marked phase of the microscopic denticulations already 

 mentioned in connection with the concentric ridges. In a still further 

 developed stage the spines cover the entire surface of the posterior field, and 

 are conical, pointed, or truncated. Notable variations of this form are seen in 

 different fishes ; for instance, in the mullet the spines are plates, with the 

 external surface raised in slightly projecting cones; but on the other hand, in 

 the perch the spines are much elongated, and appear as true spines much 

 tapered at their extremities. 



In a fourth case, as in the sole and some other Pleuronectids, the spines are 

 long, rounded, and drawn out at their extremities as in the last case ; but 

 they are not solid, but hollowed out internally into a more or less spacious 



