MUSEUM OF COMPARATIVE ZOOLOGY. 137 



Upon the dovclopinont of the scales of Tclcosts I have made no obser- 

 vations, but employ for comparison with the process in Lepidostcus 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 liold to be homologous structures, but not the homologucs of the 

 dermal spine papill;\! 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 gi-eat 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 j^art 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 aiiy 

 other gi'oup 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. 



