290 NOTES ON PALUDESTRINA JBNKINSI. 



varieties have been described. In 1890 Mr. Jenkins, with Mr. 

 Smith's sanction, considered the carinated form to be the type 

 and named the uncarinated form var. ecarinata. In i8gi he 

 described var. tnmida (Science Gossip, vol. xxvii., p. g), and in the 

 same 3'ear he proposed a var. gracilis, which, however, is fortu- 

 nately still undescribed. The shell is a very variable one, the 

 non-carinated examples passing into the carinate and the tumid 

 form into the tall slender one, so that these names are of no 

 use. Mr. L. E. Adams has considered the smooth form as the 

 type and has recorded a var. carinata, Smith, but this is an 

 obvious error {op. cit., pp. 144-5). 



In spite of the fact that this species has not been found 

 outside the British Islands, it has been suggested that it is an 

 introduction, and Mr. L. E. Adams has enunciated the theory 

 that it has come from Finland with timber. As we have 

 already shown it is a widely distributed form in these Islands, 

 and discontinuous distribution is in itself ahnost sufficient to 

 prove that it is an ancient inhabitant. In 1897 our friend Dr. 

 Frank Corner sent us a small box of shells which he had obtained 

 from a section exposed in enlarging one of the "fleets" in the 

 Roding Valley, near Barking. The shells occurred in patches 

 under two to three feet of " marsh clay." There were about a 

 dozen examples of raludestrina jenkinsi associated with Bythinia 

 tentaculata (Linn), Liiiiiia'a triincatula (Miill.), Planovhis inarginatus 

 (Drap.), and P. spivovbis (Linn.). These shells still retained 

 their periostracum, a characteristic of many ot the shells from 

 the Alluviinn. It is, of course impossible to pronounce defi- 

 nitely on the age of these shells, but they are of considerable 

 antiquity though within the historic period. Thus there can be 

 no doubt that Palndesiviiiajenkivsi has lived in the Thames Estuary 

 for a very considerable time. 



In 1859 the late Mr. G. B Sowerby figured, but did not 

 describe, a shell under the name of Rissoa castaiiea, Jeffreys^ 

 examples of which had been taken by Mr. Pickering in a ditch 

 about two miles below Gravesend.^ Dr. J. Gwyn Jeffreys, in 

 referring to these examples, states that they were considered by 

 Forbes and Hanley, though with some doubt, to be a variety of 

 Hydrobia ventrosa, but, in his opinion, since they so greatly resembled 

 a species of Hydi'ohia from the Cape of Good Hope, lie could not 



4 lilustratfd Index of British Shells, pi xiv., fig. ii. 



