SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 469 
Another conclusion to be drawn from the foregoing observations, is the corrobora- 
tion of a portion of the views of M. Agassiz in reference to the mode of growth 
which has obtained amongst the scales of “ ganoid fish,” viz. that it has been accom- 
plished by the addition of new lamellae applied to their exterior* : but these growths 
have not been confined to the lower surface ; in some, as in Le'pidosteus, Acipenser, 
and Holoptychius, they have partially covered the upper one also ; whilst in others, as 
in Lepndotus, Aspidurhynchus, &c., the concentric circle has been made complete, 
superiorly, either by the addition of continuous layers of true bone or of ganoin. 
In either of the latter cases the newly-formed lamellae have completely enclosed the 
older growths ; consequently, though agreeing with M. Agassiz in the main, I can 
scarcely conclude, as he does, that “ I’organe secreteur est la poche 6pidermoidale 
dans laquelle elles sont enfoncees par leurs bords anterieurs.” It is evident that 
each scale must have been completely and permanently surrounded by a kind of 
periosteum, closely embracing its entire circumference, and prolongations from which 
have entered many of the Haversian canals in such genera as Megalichthys, in the 
opercular bone of Lepidotus, and in Macropoma Mantelli. Though corresponding 
prolongations may have also entered the smaller parallel tubes of the Lepidostei and 
Lepidoti, we have no evidence that these latter prolongations possessed any seereting 
power, since no parallel lamellae line their vertical walls ; and in the same way, those 
portions which have lined the vertical Haversian canals of Holoptychius and the 
trumpet-shaped cavities of Megalichtliys, do not appear to have secreted any solid 
tissues. This membrane has doubtless derived its supplies of blood from the soft 
integument, which has not only been in contact with the whole base of the scale, but 
also with the superior surface of its anterior and often lateral margin. 
We also obtain from the history of these scales, some important evidence illustrating 
the process of bone-growth amongst the Mammalia. Most anatomists are aware that 
some new and highly interesting views have been advaneed by Prof. Sharpey, who 
has shown that, in the case of each human bone, but a comparatively small portion of 
it had originated from the deposition of calcareous matter amongst the cells of 
true cartilage ; all its subsequent increase in thickness or diameter proceeding from 
the calcifieation of the inner layer of the periosteum, whose fibres are always found 
to be in intimate conneetion with the osseous surface. According to this view, the 
Haversian eanals have not necessarily originated in any previous arrangement of 
the cartilage eells, but mainly resulted from the inflexions of the intermembranous 
growths of bony lamellte, formed in the substance of the periosteum without the in- 
tervention of any temporary eartilaginous structure: and as an additional sequence, 
it becomes probable that the lacunae and their canaliculi had resulted neither from 
a modification of the eartilage cell, as is believed by Schwann and Henle, nor of its 
nucleus, the view entertained by Mr. Tomes'!- ; but rather that they are little vaeuities 
* See page 437. 
MDCCCXLIX. 
t See Todd and Bowman’s Physiological Anatomy, p. 119. 
3 P 
