MUSEUM OF COMl'xVUATIVK ZOOLOGY. 121 



these thickenings arc quite distinct from one another, being separated 

 by tracts of immodified 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 Xlaatsch ('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 papilUc of tlie 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 papilhe. 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 papilho. 

 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 iu 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 

 papilkie 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 iu 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. 



Tlie 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 papillae 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 papillae of Selachians, as maintained by 

 Klaatsch and Hofer. 



llofer claims tliat in the trout the basal layer of cells of the epidermis 



