MARSHALL : ON HYDROBI^ AND ASSIMINE/£. I4I 



occurs in great numbers. There is no published figure of it, 

 but it stands in the same relation to H. similis as Faludiua 

 vivlpara does to P. conteda. Both Sowerby's and Jeffreys' 

 figures of H. siiuilis are excellent, but the dimensions given by 

 the latter are too large by one-third, and his generic figures in 

 Vol. I. of ' British Conchology ' are not so reliable. When the 

 foregoing was published in 1862 its recorded habitat was " the 

 side of the Thames from Greenwich to below Woolwich." In 

 1875, I found it had migrated down the river at least ten miles, 

 and extended onwards to Tilbury. The Rev. J. W. Horsley, 

 of Woolwich, has l)een trying my old collecting ground, l)ut 

 cannot find the species without going two or three miles further 

 down the river to Erith, so that it still appears to be migrating. 

 A clear white form that occurs with it I have named var. ca?idida. 

 There is also a form of If. ventrosa which I propose naming 

 var. carinafa, described as follows :^Shell of the var. ovata 

 form, having a sharply defined line encircling each whorl, nearly 

 in the centre. This line is not really part of the shell, but of 

 the epidermis, which is pinched up into a sharp ridge with a 

 ragged edge. Young shells also have this ridge, but as they 

 grow it wears off and appears in the adult specimen on the last 

 two or, sometimes, three whorls only. It appears much more 

 conspicuous on some specimens than on others, for as the 

 animal grows old this ridge wears down and becomes obscure 

 through wear and tear. This variety is not uncommon, and 

 occurs with the type. 



Assiviinea grayana, I would add, is also migrating down- 

 riverwards. Many years ago it was found abundantly in the 

 Greenwich Marshes, but when Dr. Jeffreys in 1868 wanted fresh 

 specimens for the purpose of illustrating his fifth volume, he 

 could find only two specimens after a most diligent search, 

 assisted by myself, and his recorded habitat in that volume was : 

 " Banks of the Thames, between Greenwich and a little below 

 Gravesend, making altogether a distance of about twenty miles," 

 but a note in my interleaved copy, written in 1872, says: " This 



