BY MOLECULAR COALESCENCE. 87 
he were not informed of its real nature.” The resemblance 
between dentine and crab-shell, as observed by Dr. 
Carpenter, is in most respects correct, and sufficiently 
striking to justify the inference that they are formed. in 
a similar manner. ‘This, I believe, is the general opinion 
of physiologists. The globular particles of carbonate of 
lime in shell are doubtless analogous to the globular den- 
tine in teeth, and the apparent spaces between the radi- 
ating lines in the former correspond in a great measure to 
the so-called dentinal tubes im the latter. But respecting 
the fact of these being in either case tubes, the results of 
my experience and investigations are opposed to this 
view. It is evident to me from the Plate XV, fig. 14, in 
the descriptive catalogue of the Royal College of Surgeons, 
that if the tubes there represented are intended to show a 
form of structure analogous to dentinal canals, as they are 
generally termed, the author has made a great. mistake. 
I may observe, that in the end of the crab’s claw, as seen 
by the microscope, there are, as in other parts, the two dif- 
ferent kinds of structure just described, one consisting of 
alternate dark and light, generally sinuous, lines, extend- 
ing from the superficial towards the deep surface of the 
shell generally considered to be tubular. These lines 
exist in every part of the shell sufficiently thick to present 
them, and, in certain positions, as in the end of the claws, 
their course gives them very much the appearance of den- 
time. However, I may repeat the observation, that, in 
the claw, especially of the very young animal, they de 
generate into mere dots before they arrive at the cavity of 
the claw, and present nothing in their appearance of a 
tubular character. The cause of this arrangement of the 
particles of the shell has been explained, and there can 
be little doubt but that this same explanation applies to 
