256 CYCLAD^. 



size, some being twice as large as the others ; the 

 larofest were about 1 1- line lonji;. 



When the siphons are very much extended, the 

 difference in the length between the two is not so 

 great as above, as it is the basal part of the siphons 

 which ajDpears to be the most extensile, the apical 

 parts keeping the same relative length to each other 

 that they did in the less extended state. 



I am informed that some British conchologists 

 consider C. pallida to be the C. lacustris of Dra- 

 parnaud ; it is very unlike the specimens I have 

 received from France and the rest of Europe under 

 that name. 



In company with the C. pallida described in the 

 last Number of the " Annals," Mr. Rowse finds 

 another species of the genus, which is very distinct 

 from the well-known and generally distributed C. 

 cornea in being subtriangular, which gives it much 

 the external appearance of a species of Pisidium. 



I cannot identify it with any of the species in the 

 British Museum collection, nor can I find any 

 description or figure representing it in any of the 

 works on European freshwater Mollusca; I there- 

 fore indicate it as new. 



It most resembles some specimens which we have 

 received as Cyclas tumida of Pfeiifer ; but I do not 

 find any species under that name in Dr. Pfeiffer's 

 work. The Paddington Canal specimens are more 

 inequilateral, longer, and more triangular, having a 

 very distinct hinder slope. 



]M. Deshayes considers C. tumida as only a variety 

 of C. cornea. 



