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 probably 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 
unlike 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 vu 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, Ill. (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 where 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 Lepidostens 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. 
T am therefore led to the conclusion that, as maintained by Klaatsch, 
the outer scale layer, called enamel by L. Agassiz, Reissner, and Hertwig, 
