4 
284 Scientific Intelligence. 
It thus corresponds with the enamel of teeth, to which it is analogous 
in every respect, save the character of the mineral secretion and the 
larger size of the prisms. 
The inner portion of shells (and the whole of many) is described as 
destitute of the distinct prismatic structure, and no trace of cells can, 
for the most part, be distinguished. This is designated the membranous 
shell structure, and instead of corresponding to epithelium or epidermic 
secretions, like the prismatic portion, it is believed to proceed from the 
true skin below the epidermis. Mr. Carpenter considers the secretion 
as having been formed within cellules, but that these cellules disap- 
peared by bursting or liquefying; a few only of these cellules remain _ 
entire which appear scattered through the layer.* 
* On polishing down the lamelle of some Astrea corals, and examining with a 
microscope, nearly the same structure may be observed, as here described by Mr. 
definite in character to be represented. By rubbing a crayon over a piece of paper 
a little rough, as good a figure of it would be had, as a more labored sketch could 
give. There were parallel bands of light and shade corresponding in direction 
with the margin of the plate, and with all its dentations, which indicated what 
other observations had shown, that these plates gradually enlarge by the extension 
of the edge ; and this was the only evidence made out of regularity of structure. 
Examined with a polarizing attach tto the microscope, the thin slices permitte 
the observatio posed of minute 
granules, confusedly aggregated, as if each had teen the result of independent 
formation, or the secretion of a separate animal cellule. There was no reason 
whatever to infer that the particles of the plate had been secreted, and accumu- 
lated by superposition, under erystallogenic forces; on the contrary the structure 
was completely amorphous, and such as could have resulted only from a mere ag- 
gregation of extremely minute granules, themselves crystalline. These lamella, it 
is well known, are internal secretions, as alsofis the whole coral. | 
The structure of the spicula in the Aleyonia appears to be somewhat different. 
These spicula are commonly five or six times as long as their greatest breadth, and 
material from the depositing secretions, With a power magnifying 300 diameters. 
i, trace of animal cellules was distinguished, and no regular texture apart from 
the evidences of a concentric structure above stated. ID. PD: 
