THE LOCAL EXTINCTION OF MOLLUSCS. QI 



Limncsa and Succinea are then at a decided disadvanta.t^'^e. I have 

 carefuUy noted the case of both species of Liiniura above men- 

 tioned. Limnaa tyuncatiila is always ready to take an excursion 

 on wet grass, but L. palustris will not go beyond the wet mud on 

 which it delights to repose ; and what is more, it does not here 

 live with other than a river-communication. Both may be 

 described as marsh species, and the drying up of marsh land 

 affects them equally. A small colony of L. palustris, var. corvus, 

 is here (at Felstead) reduced to probably less than a dozen 

 members. I saw one of these pairing with an individual of the 

 normal t\'pe — a sure sign of its diminished nntr.bers. The pond 

 fauna also, from the circumstance of increasing limitation, is 

 gradually giving in.^ Limncea stagnalis of the normal type is now^ 

 very uncommon ; the variety fvagilis is much more plentiful ; 

 both have forsaken the river. Planorbis carinatus appears to have 

 nearly forsaken the river also, although it is common in such 

 ponds as we have here. 



These instances will show that our Freshwater Molluscs 

 undergo the same mutations of distribution as their land kins- 

 men. Taking mine as a fair locality, we find that local extinc- 

 tion is in constant progress. 



We have now to turn to the other side of the question, and 

 see what we have in the way of introduction to counterbalance 

 the loss of species becoming extinct. It is obvious that wlieie 

 whole families are unable to exist on account of unfavourable 

 conditions, there is but little hope of new arrivals establishing 

 themselves, assuming these to require pretty much the same 

 conditions as the old members. Nevertheless, we are not quite 

 without examples of the immigration ot species not native of the 

 locality. 



The introduction of a species is not a matter of common 

 observation, but I have had the good fortune to witness what I 

 believe to be a great local increase of one species — Helix vivgata. 

 It came in from the north, apparently, and the great army is 

 still to be found a few miles in the rear of the outposts : but it is 

 most certainly moving southwards, and, I doubt not, will pass 

 over this part of Essex like a slow-moving wave. Then, again, 

 among the aquatic species I have, 1 think, been able to trace a 



8 [" Drainage has, no doubt, been carried out very largely during the past three or four 

 years. On the other hand, the disuse of the Chalky Boulder Clay as manure has caused the 

 marl pits to become ponds, which are extremely numerous in a C. B. Clay district — as may be 

 seen N. of Chelmsford." — T. V. H. On this Mr. French remarks, " Our marl-pits at Felstead, 

 as a rula, do not contain water, they having been dug through to the gravels."] 



