322 JOURNAI, OF CONCilOLOGY, VOI,. 12, NO. 12, OCTOBER, igog. 



There can be no doubt that these shells were obtained from the 

 gravel of the Thames, for all the specimens show what is known 

 amongst archaeologists as the " Thames mark." This is a deposit of 

 carbonate of lime which is commonly found on objects dredged from 

 the Thames or Tea, and which has probably been caused by algse. 

 This deposit was nearly half-an-inch thick on some of the shells. We 

 may, Ave think, thus conclude that at some previous period Margarita n a 

 inargaritifer was living abundantly in the Thames, and it now remains 

 for us to endeavour to fix the age. 



The gravel in which the shells occur, is not a modern deposit, for 

 it is very different from the ordinary Thames " ballast," being firmly 

 concreted together by the deposit of carbonate of lime referred to 

 above ; it is only dredged with difficulty. 



It is very clear that the present slow-moving Thames is altogether 

 unsuitable to the existence of Margaritajia, and the conditions 

 therefore must have been very different when the species lived 

 there, and from geological evidence we know that at a former period 

 the necessary conditions existed. 



At the close of the Pleistocene period the land stood at a much 

 higher level than it does to-day, and the Thames was then a quick- 

 flowing stream in a deep and narrow gorge, of which the numerous 

 borings, excavations for the piers of bridges and so forth, afford ample 

 evidence, as existing under the present channel. It is to this period — 

 the close of the Pleistocene — that we would venture to assign these 

 shells. 



The cause of the extinction of the species is easily explained by the 

 fact that as the land sank the river became more sluggish and silt and 

 mud commenced to accumulate. Such conditions would prove 

 highly detrimental to its welfare and the species soon ceased to exist. 



This record is all the more interesting as Dr. R. F. Scharff, in his 

 European Animals,'^ lays special stress on the absence of this species 

 from the south-east of England — the area so characteristic of the 

 Germanic species. 



Though the first authentic record of the occurrence of the species 

 in a fossil state in this country, it is not the first record for the Con- 

 tinent, as Locard in his Malacolo^ie Lyonnaise'^- z\\.t^ it as occurring in 

 the middle Pleistocene of Dauphine and Jura, under the specific rank 

 of Unio sinuatus Lam. 



1 London, 1907, p. 72. 



2 Lyon, 1877, p. 106. 



