Identity of Pholadidea papyracea and Pholas lamellata. 13 
and vast numbers become, as I believe is the case with all the 
Mollusca, at least the majority, the prey of the Echinodermata, 
Crustacea and other eyemies ; therefore ouly a comparatively few 
survive, to continue the race and keep up the stock diminished 
by the annual demand for them, rarely for bait, but chiefly to 
supply the cabinets of the sheli-collectors. These are the causes 
which fully account for the circumstance of twenty adult Pho- 
ladidea papyracea occurring for one in a state of adolescence ; 
thus, in conformity with the Malthusian doctrine, the ground 
being pre-occupied, no more stock can be admitted until some 
of the older colonists are removed, and reproduction is conse- 
quently limited by the ova becoming the prey of a multitude of 
enemies. 
I will say a few words on the pelagic Pholades inhabiting 
masses of stone dredged up in the littoral zones of the Devon 
coasts, six or eight miles from land. These shells, whether they 
are the two forms of Pholadidea papyracea, or the Pholas parva or 
P. dactylus, are always dwarf. I have a curious series of minute 
and completely adult Pholadidea papyracea not exceeding a 
quarter of an inch in length. Such shells are considered by the 
inexperienced observer as proofs that at all ages the Pholadidea 
papyracea is completely covered with a dome and continues gra- 
dually to increase : this is impossible, as when the dome and cali- 
ciform posterior extremity are once formed, all further growth is 
for ever terminated. The pelagic Pholades rarely exceed half an 
inch in length, consequently these dwarf forms are the result of 
locality, depth of water and many other conditions. In the deeper 
zones, the young forms of the present species, instead of being 
found in the proportion of one to twenty of the adult shells, ap- 
pear in equal numbers: this discrepancy in the proportions of 
the young shells inhabiting the littoral and pelagic zones, must 
arise from the circumstance that in the deeper waters there 1s 
more room for reproduction, more sustentation and fewer ene- 
mies; this view corroborates the doctrine above, accounting for 
the disparity of numbers in the littoral zones between the young 
and old shells of this species. I have omitted to mention that I 
possess these shells in a genuine state of transition taken by 
myself in situ, and not produced by the arts of fraudulent 
dealers. 
I terminate the present paper by stating a fact of the greatest 
importance in the ceconomy of the Bivalves, which I believe is 
not generally known, and which was discovered by me twenty 
years since, but not then promulgated, except to a few friends, 
and lately I named it to Professor Forbes: though the fact was 
new then, I do not vouch that it is so now, as from my long seces- 
sion from malacological pursuits, many of the recent discoveries, 
