MUSEUM OF compauativp: zoology. 135 



mo to show that not only in histological structure, but also in surface 

 contour and in the degree to which Haversian (or Volkmann's) canals 

 arc developed, Lepidosteus scales stand soniewhat more nearly related to 

 those of I'olypterus than has been held to be the case. 



The relation of the Ganoid scale as seen in Lepidosteus to those of 

 other groujjs of fishes remains to be considered. If my conclusions with 

 regard to this case be true, tlie idea that any (jlanoids have enamel- 

 covered scales will have to be discarded, for Polypterus scales are so 

 nearly like those of Lepidosteus as to make a difference of origin 

 extremely improbable, and no other fishes of this group have scales so 

 closely resembling enamel in physical characters. ^V'Ilat relation do the 

 .scales of Lepidosteus bear to those of Selachians 1 In the scales of these 

 two forms there is much that is unlike. In the former the basal plate 

 begins to be formed first, in the latter the spine is the part first to 

 appear ; in the former the spines are many upon each scale, and they 

 are small and transient, in the latter there is only one to a scale and 

 they are large and persistent. In the former the scale plate contains 

 osteoblasts, Haversian canals, dentinal tubules, and three crossing sys- 

 tems of incorporated fibres ; in the latter only dentinal tubules (not in 

 all respects homologous with those of Lepidosteus) and in most cases a 

 few ends of the vertical set of fibres, though in the more highly devel- 

 oped cases three sets of fibres are found. 



There is however one respect in which the two agree quite closely. 

 The spines are in all essential characteristics alike. Both have the tip 

 covered by enamel secreted by the basal epidermis cells ; both have the 

 main part composed of calcareous secretion (dentine) of dermal cells 

 lying withm and occupying a central cavity. Both have a system of 

 dendritic tubules extending from the cavity into the region of the tip, 

 and both arise by the calcification of the outside of a dermal papilla. 

 These must, then, be taken as the fundamentally homologous parts, and - 

 must serve as the basis for comparison. 



In the Selachians the simple spine has remained as the typical 

 sti'ucture, and only in the more highly developed cases (Mustelus Icevis) 

 has the basal plate been developed to the extent of incorporating in 

 itself the fibrous dermis. 



In the Ganoid scale two changes have taken place in the passage from 

 the condition in Selachians : — 



1. The basal plate has increased in size and in comi^lexity of or- 

 ganization until it has become the essential structure ; not only has it 

 incorporated m itself the dermal fibres, but with them it has also taken 



