MUSEUM OF COMPARATIVE ZOOLOGY. 135 
me to show that not only in histological structure, but also in surface 
contour and in the degree to which Haversian (or Volkmann’s) canals 
are developed, Lepidosteus scales stand somewhat more nearly related to 
those of Polypterus than has been held to be the case. 
The relation of the Ganoid scale as seen in Lepidosteus to those of 
other groups of fishes remains to be considered. If my conclusions with 
regard to this case be true, the idea that any Ganoids 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. What relation do the 
scales of Lepidosteus bear to those of Sclachians? 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 : 
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 within 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, thon, 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 
structure, and only in the more highly developed cases (Mustelus levis) 
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 complexity of or- 
ganization until it has become the essential structure; not only has it 
incorporated in itself the dermal fibres, but with them it has also taken 
