10 THE PERIODIC GROWTH OF SCALES IX GADID.-E 



stands how tliey may abruptly disappear, and how new striae may interpose 

 tlieniselves between previously existing striae. 



The grooves. These represent channels carved out of the surface of the 

 superior layer ; and the conjunctive substance mentioned above constitutes 

 their foundation. Besides the grooves directed from the periphery towards 

 the centre of the scale, there e.xist in Ophidiuiu and other fishes grooves 

 concentrically arranged. "These varied grooves may contribute to the en- 

 large uient of the scale at the surface, and permit through the intermediation of 

 the conjunctive substance, which calcifies slowly, a continued deposition of 

 calcareous salts in the lamellae, which are not in direct connection with the 

 skin, and in the conjunctive substance of the scale," 



The focus. Regarding the focus of the scale, Salbey agrees with Peters in 

 rejecting Agassiz's idea as to its formation by a process of exfoliation or 

 wearing down of the oldest layers of the scale. It is natural, he says, that 

 the projections which are nearest the centre of the scale should be smaller and 

 less apparent than those which are situated nearer the periphery, because at 

 the central point, where the superior layer of the scale is thinnest, as it was 

 formed at a period in the early life of the fish, the projections or crests would 

 not be so distinct and pronounced as those in peripheral parts formed during 

 the later life of the fish. The presence of skin at the surface of the scale does 

 not allow of any other explanation, and in order to believe that a wearing 

 down has there taken place, it is necessary to suppose that a destruction of the 

 ejiidermis and of the skin has taken place at this part. The focus is nothing 

 else than the oldest part of the scale. It is also the thickest part of the 

 scale, because there we have the greatest number of lamellae at the internal 

 face of the scale. 



Corpuscles. Salbey does not bring forward any important facts as to the 

 corpuscles of scales. He agrees with Leydig in regarding them as ossified 

 clobular bodies. 



Teeth. Regarding the teeth of scales, Salbey rebuff's the opinion of Peters, 

 according to whom these parts develop from the corpuscles of scales. He also 

 disagrees with Mandl, who regarded these appendages as true teeth. He 

 considers these small teeth as integral parts of the superior layer. These 

 minute teeth appear successively at the posterior border of the scale as that 

 grows ; it is because of this mode of growth that the points formed in the last 

 part appear perfectly preserved, while those which during the progress of 

 growth become carried further forward are very small and much broken by 

 external friction. 



Carpenter devotes several pages to the structure of scales in osseous, ganoid, 

 and placoid fishes.* On the subject of osseous fishes, he deals in a very 

 concise manner with the scales of the eel, carp, and sole. 



Regarding the cycloid and ctenoid divisions established by Agassiz, he 

 considers this sharp division as having little harmony with the general 

 organisation of the types which it has the aim of separating. 



Yaillant also takes up the question of the value of cycloid and ctenoid 

 characters as propounded by Agassiz for purposes of classification.! He shows 

 the great variation which occurs in the scales of Percidae, not only in different 

 * Carpenter, 1868. t Vaillant, 1872. 



G 



