80 FORMATION OF SHELLS OF ANIMALS, ETC., 
them whilst they are under examination by the micro- 
scope, the escape of gas, as seen passing from them, 
indicating the presence of one of their components— 
carbonic acid. After thus acted upon, as was noticed of 
the artificial analogous form of carbonate, they leave a 
residue of soft material— animal basis’’—of the same 
shape and appearance, but much less distinct. On the 
membranous layer next to this, and on those still nearer 
to the surface, these particles can be seen in every stage 
of coalescence, presenting so exactly the appearance of 
those on the slide*employed in the artificial process, as 
not to admit of beimg distinguished from them. The 
resemblance between these and the artificial products is, 
in some respects, more complete than between the latter 
and the urinary calculi of the horse. In the shell all the 
different forms and stages of coalescence are better marked, 
and therefore more distinct than in the calculi from the 
horse’s urine; and in those of the shell, which are not so 
well marked as the artificial ones, it will be evident that 
the process of coalescence has been interfered with by the 
mechanical causes, and that just in proportion as in their 
formation they can be seen to have been exposed to me- 
chanical interference from the contiguous structures, so 
do they suffer distortion and become less like those pre- 
pared artificially. Consequently, the smaller these glo- 
bules are, the more exact is their resemblance to the 
artificial ones of the same size. By varying the artificial 
process also, the less perfect forms of the natural products 
can be accurately imitated. See fig. 5, which is an accu- 
rate representation of a portion of the innermost layer of 
the shell of a very young lobster, about four mches in 
length, in which globular particles, of various sizes and 
in all stages of coalescence, are shown. In the shells of 
