14 THE NAUTILUS. 



characterized by the smooth or striate apex. In differentiating the 

 species in these groups, the shape and contour of the shell are the 

 main elements to be relied upon, the sculpture of the surface being an 

 exceedingly variable factor, which, by itself, cannot in most cases be 

 considered a specific character. As in all fresh-water forms, a very 

 large degree of variation must be allowed for. But in spite of this, 

 it is believed that nearly all the described species should be allowed 

 to stand, and, although in certain instances it is not always possible 

 to determine the exact specific relations of particular specimens, yet, 

 as a rule, the lines between the different forms can be drawn with a 

 reasonably satisfactory degree of certainty. 



Bourguignat, in his " Notice stir le genre Ancylus," in 1853 (J. de 

 C. IV., p. 63), divided the genus into two subgenera : Ancylastrum, 

 with the apex inclined to the right, and YeUetia, with the apex in- 

 clined to the left. 



Clessin (1882), in the Conchlien Cabinet, considered these two 

 groups to have only a sectional value. He also restricted Ancylas- 

 trum to the Eurasian species which group around A. fluviatilis ; and 

 with the exception of A. fragilis and oregoneiisis, which he referred 

 to Velletia, and the large western A. neu'berryi and patelloides, which 

 he placed in a neAV genus, Lanx, included all the North American 

 species in a separate group, Haldemania, which he characterized as 

 follows : " Shell conical, apex not bent backwards and only slightly 

 removed from the centre-line of the shell, aperture round or oval. 

 Type A. obscurus Hald." 



Unfortunately Haldemama is preoccupied, having been used by 

 Tryon in 1802 (Proc. P. A. N. S., 1862, p. 95) for the group of 

 Viviparidse now known as Lioplax, so that his very appropriate 

 name cannot be retained. 



As has already been shown (NAUTILUS, XVI., p. 85), the North 

 American species included in Clessin's Haldemania are divided into 

 two natural groups, characterized by the presence or absence of apical 

 sculpture. These groups are, at least, of sectional value, and must 

 be recognized. 



Owing to the uncertainty which still prevails as to just what 

 Haldeman's obscurus really is, and the consequent inability to say 

 with accuracy to which group that species belongs, it does not appear 

 desirable to retain obscurus as the type of either section. Whenever 

 an examination of Haldeman's type shall definitely determine where 



