356 Messrs. Hancock & Atthey on Reptile- and Fish-Remains 
account of the microscopical characters of these peculiar dental 
plates. 
In sections made across the transverse ridges that cover the 
whole surface of the plates, a very beautiful structure is pre- 
sented to view. The entire substance is found to be com- 
posed of a minute reticulation of bone-like matter, the meshes 
or medullary canals being large and much complicated. The 
ridges stand up from the surface in the form of conical tooth- 
like processes; and the reticulated matter of which they are 
composed is perfectly continuous with that of the plate or 
base ; but the meshes or medullary canals in them are a little 
elongated, and the surface is protected by a compact, rather thin 
layer, which is only distinguishable from the rest of the tissue 
by its density and darkness of colour; on this layer there is a 
thin external coating of enamel. 
At the base of the plate there is a stratum of considerable 
thickness in which the reticulation becomes somewhat closer, 
and which is characterized by numerous short elliptical bone- 
cells, the radiating canaliculi of which are frequently oblite- 
rated, but in well-preserved specimens they can be observed 
distinctly. The network of this stratum is continuous with 
that which lies immediately above it, but is at once distin- 
guishable by its darker colour, greater density, and the pre- 
sence of radiating cells. The substance forming the reticula- 
tion of the upper portion of the plate is, on the contrary, de- 
void of bone-cells, and is pale and transparent; but it is coated 
with a thin layer of a darker matter, in which are numerous 
branched tubules. When the section is made very thin, these 
tubules, however, all disappear, and the substance is then to 
all appearance perfectly homogeneous. These tubules are 
likewise very frequently invisible, even in. comparatively thick 
sections, probably on account of the state of the fossil; or it 
may be that the canaliculi have all disappeared under the in- 
fluence of the balsam used in mounting the specimens. 
The peripheral enamel is very often wanting ; and even the 
dense continuous layer of bone-like matter immediately be- 
neath it is frequently entirely worn away; and then the section 
presents a rugged margin. 
The microscopic structure of Ctenodus has been figured and 
described by M. Agassiz, in his ‘ Poissons Fossiles’ (vol. iii. 
p- 166, tab. M. f. 3). The figure is very good, so far as it is 
worked out; but when the author describes the ‘ cellules 
calciféres” at the base of the plate as without ramifications, it 
is evident he has been deceived, probably by the use of bal- 
sam; or it is just as likely that the canaliculi had not been 
preserved in the specimen he examined. He is also wrong in 
