Identity of Pholadidea papyracea and Pbolas 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 enemies ; therefore only 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 shell-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 is 

 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 oeconomy 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, 



