JEFFERY: the MOLLUSCA of western SUSSEX. 307 



county, residing in the mud at the bottom of ditches 

 and slow streams. The upper portion of the valves, that 

 above the mud surface invariably incrusted. 

 P. fontinale. The most common and generally distributed of 

 the genus. Last April while examining one of this 

 species under the microscope I witnessed the birth, at 

 intervals, of three young through the siphon. The foot 

 of the young bivalve always appeared first, clinging 

 round the edges as if feeling its way out, but each one 

 was ultimately expelled by a current .or rather blast of 

 water by the parent, and was soon starting on a voyage 

 of discovery of its own. 



With regard to microscopic observations, I think I 

 an correct in stating that I have detected in this species 

 at the edge of the mantle at the anterior* margin (around 

 the foot when extended) where the in-current of water 



*I felt somewhat puzzled as to whether I should write anterior or posterior 

 here. Authors do not seem to be agreed on this point. Jeffreys writes (Brit. 

 Conch., vol. i., p. 16) with regard to the genus Pisidium: ^'beaks placed near 

 the shorter or anterior end." But in Woodward's Manual of the Mollusca, 

 2nd edition, 1871, p. 462, is written: "anterior side the longest." This latter 

 description is in my opinion correct. 



In the Pisidiadce the foot is extended from the longer end or side, and this 

 end is to the front when the animal is travelling with the siphon at the shorter 

 end, behind. The same occurs in the genus Sphczrium^ though in this case 

 the shells are nearly equilateral. In Unio and Anodoitta however the loot is 

 protruded from the shorter end and the siphons are placed at the longer and 

 thinner end ; here therefore the shorter and thinner end would be naturally 

 anterior. 



I find in Popular Conchology, Catlow, 1854, in an explanation of terms: 

 "Anterior — The side of the shell, in Bivalves, where the ligament is 

 situated," and "Posterior" otV^ z/^rj^. It is probably on this ground that 

 Jeffreys makes the assertion above alluded to, but if this be accepted as correct 

 we must decide that Sphmrium and Pisidium travel backwards or posterior end 

 first. Therefore I have in this case followed Woodward and written anterior. 



The fact of the in-current in these genera being taken at the anterior 

 margin, which margin would when the moUusk is under the surface of the mud 

 be the part most deeply buried, the siphon being uppermost, tends to the 

 inference that they derive their food from beneath the surface of the mud, or it 

 shows that they are better adapted for a life above it than either Unio or 

 Anodonta. 



