PISIDIADJE. 263 



higher up the same river, on stones of a small over- 

 fall at Tansor: he believes they were introduced 

 from Wisbeach on timber since 1828. 



It has been naturalised in Holland and on the 

 Rhine. It is also found with tertiary fossils in 

 Transylvania, Moravia, and near Vienna. 



Mr. Lyell (Geol.\ not being aware that these 

 animals had the power of living a long time out of 

 water, and that they were most probably brought in 

 the holds of ships with the Baltic timber, and thus 

 introduced into our docks, where the timber is un- 

 loaded, believes that the animals attached themselves 

 to the bottoms of Baltic ships, and in this manner 

 were obliged to pass through the sea, before being 

 again brought to their natural station in fresh water. 

 And Mr. Garner, in his curious but rather crude 

 paper on the anatomy of Lamellibranchiata (Mag. 

 Nat. Hist. n. s. iii. 303.), ventures to explain this 

 theory by supposing that the animals " kept their 

 valves constantly closed " during the voyage through 

 the sea to the fresh water ! 



ORDER III. LASIACEA. 

 Fam. 3. PISIDIADJS. 



Mantle lobes united; pedal and branchial opening 

 inferior ; syphon conical, more or less elongate ; 

 foot lanceolate, compressed, not byssiferous ; 

 s 4 



