170 BRITISH MOLLUSKS. 



respiratory and other organs on the right side of the animal, and 

 the vertex of the shell inclines to the left. The animal is wholly- 

 covered by the shell. In our vignette, from a drawing made by the 

 Eev. M. J. Berkeley, the front of the shell is lifted to show the 

 animal, which it may be seen has the broad head and slender sub- 

 triangidar tentacles of Physa, with the eyes situated as in that 

 genus at their inner base. 



The geographical distribution of Ancylus is curious. In Europe 

 we have probably only a single species of each of the groups, A. 

 fluviatilis and lacustris. More than a dozen species have been 

 described as European, but they appear to me quite untenable. 

 Local modifications of form, substance, and colour cannot be sus- 

 tained as specific characters. The only other species of Ancylus 

 known in the Eastern Hemisphere are one in Siberia, one in Tas- 

 mania, one in New Zealand, one in Natal ; and one in TenerifFe 

 appears to be distinct from the Madeiran form of the European 

 species. In the Western Hemisphere Ancylus has a well-esta- 

 blished centre of geographical diffusion in the West Indies and 

 Central America. Southwards it ranges to Venezuela and Bahia, 

 and to Mexico and Chili. Northwards it reaches over California 

 and the United States to Oregon and Newfoundland. About thirty 

 species, reducible apparently to twenty-four, have been described 

 from this wide- spread region. 



The dextral form of Ancylus (Velletia) has the same range of 

 geographical distribution as the sinistral (Ancylastrum), with the 

 important additional habitat of Bengal (A. Baconi), but it is scarcer 

 in species and more limited in individuals. It has been stated that 

 the Velletia form of Ancylus has not been found out of Europe, 

 but I consider the A. Barilensis, inhabiting Lake Baril, Bahia, 

 A. Haldemanni, from Massachusetts, A. Verreauxi, from the Cape 

 of Good Hope, and an undescribed species from Mexico in Mr. 

 Cuming's collection, as belonging to this group. No Ancylus has 

 been found among the Lymnceacea of the Malayan streams. 



The British Ancyli are : — ■ 



1. fluviatilis. Shell rotundately ovate, elevated, radiately stri- 



ated, with the vertex quite posterior, incurved to the right. 



2. lacustris. Shell oblong-ovate, rather depressed, smooth, with 



the vertex posterior to central, incurved to the left. 



