130 BULLETIN OF THE 



appears in this way. That the disappearance of the spines is due to 

 resorption was suggested by Hertwig ('79, p. 8). 



The ganoin layer begins to be formed first over the central area of the 

 scale, and covers all but a very narrow marginal zone, as a regular coat- 

 ing of uniform thickness. At this time the spines have mostly disap- 

 peared from all except the posterior margin. The ganoin is separated 

 from the bony part of the scale by a distinct, regular line, and shows 

 markedly different optical properties. It is more highly refractive than 

 the part below, and appears entirely homogeneous except for a very 

 delicate striation parallel to the surface, which j^robably corresponds to 

 irregularities in the rate of deposition. It cannot be seen in cut 

 sections, since it is entirely destroyed by decalcification, without 

 which section cutting is impossible in scales of this age. In sections 

 prepared by grinding it presents the appearance seen in Plate II. 

 Figure 15. As seen from the surface, too, the appearance is quite 

 mil ike that of the bony part of the scale, which is not covered by it 

 (Plate IV. Fig. 27). A knowledge of this layer can be obtained only 

 by combining the results of study both of sections from decalcified 

 tissue and of preparations made by grinding undecalcified scales of the 

 same stage of development. 



I find the ganoin layer first present in a fish 52 months old (30 cm. 

 long) reared in confinement ; it is also present, and a very little thicker, 

 in a fish of the same length from Quincy, 111. (Plate II. Fig. 15). In the 

 former case the relation of the scale to the epidermis could not be made 

 out, but in the latter all the tissues were well preserved in a perfectly 

 healthy condition, and the epidermis was distinctly separated from the 

 scale by a thin layer of dermal scleroblasts seldom over two or three 

 cells thick (Plate II. Fig. 16, and Plate IV. Fig. 23). The former of 

 the two figures cited shows about an average condition, and the latter a 

 place near the opening of one of the canals which pierce the central 

 part of the scale whei'e the blood-vessels and surrounding tissue make 

 an unusually thick sub-epidermal sheet. Small blood-vessels are abun- 

 dant in this layer. 



On the scales of a young Lepidosteus 44 cm. long, in which the 

 layer of ganoin had a thickness approaching that found in the adult, 

 the sub-epidermal layer still persisted over the scales of both the 

 ventral and the dorsal regions, though in the latter the epidermis 

 showed a perceptible decrease in thickness. 



I am therefore led to the conclusion that, as maintainerl by Klaatsch, 

 the outer scale layer, called enamel by L. Agassiz, Peissner, and Hertwig, 



