MUSEUM OF COMPARATIVE ZOOLOGY. ys 
Upon the development of the scales of Teleosts I have made no obser- 
vations, but employ for comparison with the process in Lepidosteus the 
accounts given by Klaatsch and Hofer. 
In the earliest stages of development a similar modification of the 
dermis takes place in both cases, giving rise to local thickenings of this 
layer within which the scale begins to be formed as previously described. 
These I hold to be homologous structures, but not the homologues of the 
dermal spine papille of Selachians. 
The resulting bony plates formed in the two cases are homologous but 
that of Lepidosteus attains to a much higher degree of development 
than the one formed in Teleosts. The part to be formed first in the 
Teleost scale is, as in Lepidosteus, the outer more homogeneous part, 
but in many cases it differs from the corresponding part of the scale of 
Lepidosteus in the absence of enclosed osteoblasts. That these are 
present in some species and absent in others which are very near 
relatives shows that this difference cannot be of any great morpho- 
logical importance. Whether they have been secondarily acquired in 
one case or secondarily lost in the other need not concern us here. 
Their absence in some cases cannot prevent the layer being considered 
the homologue of the corresponding layer in Lepidosteus. 
The deeper fibrous part of the Teleost scale is the later formation, and 
in this respect, as well as in general structure and method of develop- 
ment, agrees with the deeper part of the scale of Lepidosteus. 
The outer layer of the Lepidosteus scale, the ganoin, which is the part 
latest formed, is absent in the scales of Teleosts. It is a layer which has 
been developed within the order of Ganoids, and is not found in any 
other group of vertebrates. 
Throughout the series of scale structures beginning with the Selachian 
type there has been a constant tendency toward reduction of superficial 
parts (spines) and increase of the deeper parts which are independent of 
the epidermis. In Selachians the process of scale formation begins at 
the surface of the dermis just beneath the basement membrane. In 
Ganoids there is the same process repeated at the base of the epidermis, 
but in a much less vigorous manner, while the principal activity is 
deeper-seated, in the midst of the dermis. In the higher Teleosts the 
whole scale growth is within the dermis, and the more superficial 
process is entirely lost. 
Thus I believe that the basal plate of the scale of Lepidosteus and the 
Teleost scale have both been derived from the basal plate of the placoid 
scale, and have for the most part been modified along the same lines. 
