MUSEUM OF COMPARATIVE ZOOLOGY. ivAl 
these thickenings are quite distinct from one another, being separated 
by tracts of unmodified tissue; thus each one resembles a very much 
flattened papilla. 
A similar condition found in the early stages of the formation of the 
scales of Teleosts is described by Klaatsch (’90, p. 159) and by Hofer 
(90, p. 111). Both of these authors regard these dermal thickenings 
in Teleosts as homologous with the dermal scale papillee of the Selachians. 
This view receives a certain amount of support from the fact that in 
Selachians, Ganoids, and Teleosts the first indication of the formation of 
scales is the appearance of local elevations of the dermis in the form 
of papille. On the other hand, it is difficult to imagine any change 
due to increased local activity of the tissue taking place in the dermis 
without producing modifications resembling to some extent papille. 
That the scales of all the groups of fishes owe their origin primarily to 
the dermis seems therefore a sufficient explanation of the early appear- 
ance of these papilla-like local modifications of this layer, and not to 
require the assumption of any direct homology between the resulting 
structures. The fact that in Selachians the scale is formed over the 
surface of the papilla, while in Ganoids (Lepidosteus) and in Teleosts it 
arises in the midst of the mass of cells forming the elevation, is a 
fundamental difference not to be overlooked. Furthermore, at a later 
period in the formation of the scales of Lepidosteus there arise dermal 
papillz just beneath the basement membrane, over the surface of which 
calcareous material is laid down to form spines. These receive an ecto- 
dermal enamel secretion over their tips, and later pierce the epidermis, 
and thus exactly resemble in all important features the spines of the 
placoid scales of Selachians. The formation of the spines in Lepidos- 
teus has been described already by Klaatsch (’90, p. 130), and will be 
treated of more fully further on in this paper. 
The condition found in Lepidosteus is intermediate between that 
presented by Selachians and that described in Teleosts. The presence 
of the spines enables us to say that the papille by which they are 
secreted, not the broader, earlier formed dermal thickenings, are the 
homologues of the papillae formed in Selachians. As the early con- 
ditions found in Teleosts so closely resemble those in Lepidosteus, there 
can be no choice but to interpret these dermal thickenings as homolo- 
gous with those of Lepidosteus. They cannot therefore be considered 
the homologues of the scale papille of Selachians, as maintained by 
Klaatsch and Hofer. 
Hofer claims that in the trout the basal layer of cells of the epidermis 
