298 BULLETIN OF THE BUREAU OF FISHERIES. 



addition of layers to the internal face. On the superior surface it is loosely connected. 

 Subsequent calcification is from the exterior toward the interior and from the periphery 

 toward the center. 



Nickerson (1893) says: 



Tliroughout the series of scale structures, beginning with the selachian type, there has been a con- 

 stant tendency toward tlie reduction of the superficial parts (spines) and increase of the deeper parts 

 which are independent of the epidermis. * * * In the higher teleosts the whole scale growth is 

 within the dermis and the more superficial process is entirely lost. 



Klaatsch (1894) divides the scale into the outer homogeneous layer and the inner 

 fibrillar layer. The outer layer is bony tissue, entirely soluble in hydrochloric acid 

 and having no special structure except a slight layering. It is formed from cells located 

 chiefly in the lower surface of the overlying scale pocket. The scleroblasts (formative 

 cells that give rise to scales) form the superficial reliefs. This exists for a long time 

 before the inner layer appears. 



The inner, less calcified layer consists of fibers in bundles, all the fibers in one 

 bundle being parallel, and the bundles being parallel with each other but crossing those 

 of the next higher and lower layers at acute angles. This is considered the connective 

 tissue layer of the scale. 



In the scleroblast layer there are polygonal elements between which there is a color- 

 less network. The cells in this layer arrange themselves in groups whose nuclei come 

 to lie closer together. Then those parts of the cells farthest from the nuclei separate 

 and are added to the intercellular substance. This substance is added to the part of 

 the cell already existing. The scale is thus an intercellular secretion which is eventually 

 hardened by lime salts. 



Ussow (1897) concludes his paper as follows: 



The scale of teleosteans is a plate consisting of two layers. The upper layer (including the relief) 

 consists of a homogeneous tissue without any structure except a very slight striation parallel to tlie 

 upper stxrface. This layc'r originates in the dermis at the expense of the so-called scleroblasts. * * * 

 The tissue of this layer is a simple bony tissue. The lower layer also originates at the expense of the 

 same elements. It is formed in part out of the indurated connective tissue. 



Tims (1906) found that in the cod the calcareous material does not form an unin- 

 terrupted layer, but is in the form of minute isolated platelets the exterior surface of 

 which bears a small spine resembling very small placoid scales. 



CLASSIFICATION. 



The first attempt at classification by means of scales was that of Heusinger (1823)." 

 He devised the following plan : 



1. Fishes with scales entirely hidden in the skin: Anguilla, etc. 



2. Those with scales proper: Esox, Salmo, etc. 



3. Those with strongly toothed scales: Chaitodon. 



4. Those with osseous scales: Lepidosteus, etc. 



5. Those with osseous plates: Acipenser, etc. 



6. Selachians. 



a Thomson, 1904. This reference not verified by writer. 



