332 JOURNAL OF CONCHOLOGY, VOL. I3, NO. II, JULY, I912. 



Mediterranean shell, from which this variety was described, the spire 

 is so depressed as to be out of sight when the mouth is towards the 

 observer, and the top of the aperture is level with the crown ; in 

 most of those from Guernsey the spire is more or less visible though 

 depressed, and the length of the aperture is variable, but many 

 correspond to the Mediterranean form. According to Monterosato, 

 this is " U. tnimitissimus H. Martin, ex typo." 



There are few British shells more variable than U. obtiisus, either 

 as regards size, shape, solidity, length of spire, or of aperture ; but 

 it consists mainly of two principal forms, from which all the others 

 radiate : the type form, which lives in brackish bays and estuaries, 

 and the other an exclusively marine form, inhabiting many parts of 

 our coasts from low water to 30 fathoms. The latter, besides being 

 very much smaller, differs from the former in not being constricted 

 in the centre nor broader at the base, but has an oblong outline, and 

 gradually merges into the var. lajonkaireana. Both forms are equally 

 variable, and between the two many varieties might be defined. (An 

 exactly analogous case occurs in the estuarine Hydrobia ulvce and its 

 marine var. minor). In the type, the aperture occasionally exceeds 

 the length of the shell, and the very young resemble the same stage 

 of U. truncainlus, but the latter has a hollow or depressed crown, 

 while the former is truncated only. Gwyn Jeffreys' ge?ieric figure is 

 the type of the estuarine form and of his description ; his type figure 

 is different, and too cylindrical ; Sowerby's is perfect; while "British 

 Mollusca " contains several good figures of its variations. 



The var. lajonkaireana is markedly distinct from the var. minor, 

 though equally variable ; in all its forms, and at all stages of growth, 

 it has a shorter aperture and a longer spire ; but care must be taken 

 not to confuse the young with the same stage of U. nianimillatus ; the 

 two are much alike, though the latter is somewhat more slender and 

 the apex or nipple is larger. Jeffreys' is the best figure, but is wrongly 

 inscribed var. lajojikairianus ; Sowerby's figures are equally good, 

 though the shell is usually more slender. 



There are several dwarf varieties besides the var. niifior. Jeffreys 

 has described one from the Roach River, Essex, as var. seniistriata, 

 which has longitudinal striae at the summit as in U. truncatnlus ; 

 Brusina has described another from Dalmatia, with a depressed 

 spire, as Cylichna leptoneileina ; and a remarkable form from 

 Guernsey, Scilly, and Barra might readily pass for half-grown U. 

 truncatnlus var. pelliicida ; they only differ in the latter having the 

 apical rim sharper and the crown depressed ; they are oblong in 

 shape, the spire is flattened on a level with the crown, and the aperture 

 extends considerably above the apex. 



