SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 455 
Each areola has, in the first instance, temporarily presented a similar appearance as 
is permanently exhibited by some species oi Diplopterus and Holoptychius. The most 
superficial portion has been separated from the textures below by a large horizontal 
cavity, into which the numerous extensions of the Haversian canals destined to form 
the cul-de-sacs have opened. This superficial chamber has only been traversed by 
the hollow pillars of kosmine surrounding the descending trumpet-shaped cavities. 
The framework of the kosmine has been penetrated by a multitude of exceedingly 
minute tubuli, opening into the diffused chamber. As new internal organizations of 
bone have filled up the Haversian canals, prolongations from the osseous lamellae 
have also contributed to fill up this open space, by thickening the walls of the hollow 
pillars and their narrow tubular canals, as well as the uppermost layer which supports 
the ganoin. The increase of these depositions has led, after a time, to the closing up 
of the channels communicating between contiguous areolae, a small aperture only 
being left permanently open, constituting the network of narrow tubes, fig. 16 g. 
The concentric walls of the hollow pillars, fig. 16«, thus becoming confluent, the 
central cavity of each areola has been isolated ; and further organizations have nar- 
rowed its dimensions, until nothing remained of each originally large space but the 
permanent cul-de-sac, into which the minute tubes, gradually uniting to form the 
larger branches penetrating each succeeding lamella, have ultimately opened. 
This process has gone on with the increasing age of the fish, until in some instances 
the cul-de-sac is nearly obliterated, leaving only a narrow vertical tube or stem, 
supporting the arborescent arrangement of tubuli. In some of my sections these 
lamellae are beautifully distinct, the innermost ones following the outline of the cul- 
de-sac, and the outer ones that of the trumpet-shaped cavities around which they were 
deposited. This arrangement is represented in the areolae of fig. 15. We should in- 
fer that in young fish the cul-de-sacs of the kosmine and the Haversian canals of the 
bone would be very large in proportion to the solid tissues of the scale, and I can 
even believe it possible that scales of a very young individual might easily be mistaken 
for those of a Diplopterus. I possess sections of the latter which exhibit precisely 
this condition. The Haversian canals appear as very large cancellated cavities, and 
the structure of the kosmine also resembles the early state Megalichthys SiE just 
described. 
There appears to be a period in the history of the scale when its kosmine ceases 
materially to increase in its superficial diameter. This extension seems to have 
reached its limit when the deep grooves which mark the boundary of the kosmine on 
two sides of the scale are formed. Further additions of bony substance continue to 
be made inferiorly, as well as to the upper surface of the two margins, which, being 
imbedded in the soft integument, need neither ganoin nor kosmine ; these being the 
portions where the development of the two latter is arrested by the groove. Whether 
or not any additions continue to be made to the other two margins which overlap the 
contiguous scales behind and beneath, I have not been able to satisfy myself. 
3 N 2 
