298 PROCEEDINGS OF THE HALACOLOGICAL SOCIETY. 



The opinion has been expressed that P. Jenlcinsi is identical with 

 P. crystallina, Pfr., but the Eev. Prof. H. M. Gwatkin informs 

 us that the radulse differ. He writes: "In the central tooth 

 P. crystallina has a narrower form, a more decided anterior concave 

 sweep, and more conspicuous basal teeth nearer to the posterior 

 edge. Eut the central denticle is less developed than in 

 P. Jenlcinsi. So far the specific distinction is clear. My doubt is 

 that I have a strong impression that one or the other is veiy 

 variable." ^ The non-carinated examples of these species are certainly 

 very near each other, but in the carinate specimens the carinas in 

 P. Jenlcinsi are by no means so pronounced as in P. crystallina, and 

 in the latter species they may rather be described as a series of spines 

 which thus contrast with the tufts on the former. Hence we consider 

 that P. Jenlcinsi is a good species, and not to be ranked as one of the 

 numerous synonyms of P. crystallina. 



It was originally suggested by Mr. Jenkins^ and Mr. "W. Crouch^ 

 that the species is not truly indigenous, but has been introduced from 

 abroad, whilst Mr. L. E. Adams has suggested that it may have been 

 introduced with timber from Finland, ■* That it has hitherto been 

 undetected abroad rather militates against the introduction theory, 

 and no one has suggested so far that Assiminea Grayana (Leach), 

 which is practically confined to the Thames estuary, is also an 

 involuntary immigrant. 



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 Koding Yalley, near Barking. The shells occurred in 

 patches under about two to three feet of ' marsh clay,' which deposit, 

 Dr. Corner states, is in age " within the historic period." There 

 were about a dozen examples of Paludestrina Jenlcinsi associated with 

 Bytliinia tentaculata (Linn.), Limnma truncatula (Miill.), Planorlis 

 marginatus, Drap., and P. spirorlis (Linn.). These shells still retain 

 their periostracum, a characteristic of many of the shells from the 

 alluvium. The obvious conclusion is that the species has lived in this 

 country for a much longer period than has hitherto been considered to 

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

 describe,^ a shell under the name of Rissoa castanea, Jeffreys, examples 

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

 miles below Gravesend. J. Gwyn Jeffreys, in alluding to these shells, 

 states that they were considered, though with some doubt, to be a variety 

 of Uydrobia ventrosa by Forbes and Hanley, but that they so greatly 

 resembled a species of Uydrobia from the Cape of Good Hope that he 



Cf. L. E. Adams : The Collectors' Manual, 2nd ed., 1896, p. 145. 

 "Distribution and Habits of the British Hydrobice'''' : Science Gossip, vol. xxiv 



(1890), p. 106. 

 " Note on Uydrobia Jenkinsi " : Essex Nat., vol. iv (1890), p. 213. 

 "A Theory as to the possible Introduction of Hydrobia Jenhinsi'''' : Journ. 



Conch., vol. vii (1893), pp. 148-lo0. 

 Illustrated Index of British Shells, pi. xiv, fig. 11. 



