158 



PISIDIUM PARVULUM Clessin IN THE GREAT OUSE AND 

 THE SEVERN. 



By C. OLDHAM. 



fRead before the Society, October 12th, igiS 



In some material collected in the Great Ouse at Newport Pagnell, 

 Bucks., in August last I found several dead shells of P. parvulum 

 associated with a7nnicum, casertaniim, siibtnincatmn, supininn, hens- 

 lowaniivi and ?iitidum, a typical river assemblage ; and at Bromham, 

 Beds., some 35 miles further down the stream I collected other dead 

 shells oi parvuluvi and one living specimen. The associated species 

 here were the same as at Newport Pagnell, but with jnilium and 

 hibernicuvi (a single specimen) in addition. In the river bank near 

 Bromhall mill, about three feet below the present level of the 

 meadows a band of shell-marl of Holocene age is exposed, which is 

 rich in moUusca. I detected forty-one species of land and fresh-water 

 shells, and washed out a considerable number of valves of parvulum 

 and supinum. The associated Pisidia were the same species that 

 were living in association in the river, except hiberniciim, and included 

 in addition personahmi. The last named had probably not been 

 associated with the others in life, but had been swept down 

 stream with the heterogeneous collection, wliich included terrestrial 

 species, such as Vertigo pygmcea, Carychmm minimum, Cochlicopa 

 lubrica, Vallonia excentrica and Hygromia hispida, until it came to 

 rest in the slack water at the place where the deposit was being 

 formed. 



During September I was in Worcestershire and collected some 

 Pisidia in the Severn. I failed to find parvulum at Bewdley, although 

 supinum, with which it is so often associated, was common there ; but 

 at Stourport I took a single living specimen, the associated species 

 being the same as in the Ouse at Newport Pagnell, and, incidentally, 

 the same as I have found in many places in the Grand Junction 

 Canal in Bucks, and Herts., i.e., at/micu?n^ casertamim, subtru/icatum, 

 supinum, henslowanum and nitidum. A gathering made in the City 

 of Worcester included half-a-dozen living parvulum, the associated 

 species being the same as at Stourport and Newport Pagnell, with the 

 omission of nitidum. 



P. parvulum has presumably been a member of our fauna since 

 Pleistocene times ; it is abundant in the brick-earth of that age in the 

 Thames valley, and indeed outnumbered all the other Pisidia 

 together in some material I obtained at Crayford, Kent, a year or 

 two ago. Its occurrence in a living state in the Ouse and Severn 



