On the Melanians of Lamarck. 23 



granted that the two are nearly allied, and discuss the question of 

 the identity of Melanopsis with Melania. If they are connected, 

 it must be by means of Anculosa, which stands between both. 

 The head, neck, and foot of the Melania?, are protruded to a con- 

 siderable extent ; they inhabit rivers, in running water, and are 

 continually moving from place to place, but they are not found in 

 the ripples or more rapid parts of the stream. The Anculosa?, on 

 the other hand, live attached to stones and rocks in the most rapid 

 waters ; they are of sedentary habits, seldom moving, except to 

 leave the water occasionally, by climbing up a wet rock. The 

 animal extends but little beyond the shell, as it would be liable to 

 receive injuries from the rapidity of the current ; the foot is very 

 small, discoidal in shape, and adapted to enable the animal to hold 

 with great tenacity. They are thus separated from Melania by 

 habit and structure, and a short shell is necessary, to prevent them 

 from being forced from their position by the current ; which 

 would of course, have a greater hold upon a long shell. On this 

 account T think it probable that Pirena atra bears the same anal- 

 ogy to the typical Melanopsides, that Melania does to Anculosa ; 

 and as these differences could not be well distinguished in dead 

 and contracted specimens, the fact that Ferussac referred the for- 

 mer to Melanopsis proper, is of little account. I accordingly 

 adopt this species as the type of the genus Pirena, as Lamarck 

 and Cuvier have done ; and retain it among the Melanians, be- 

 cause the mantle is not fringed, but merely sinuated according to 

 the outline of the aperture, as described by Ferussac. 



" Pirena aurita" is not congeneric with Pirena (atra) nor Me- 

 lania, but must be placed in the family CerithinEe ; next perhaps 

 to Potamis. The type is distinguished from the typical Melanias 

 by the tubercles, and the sinuated labrum ; and it cannot be sep- 

 arated from its American representatives, Melania undulata. Say, 

 and similar shells, which, although the characters are less highly 

 developed, have been characterized as distinct by Rafinesque 

 and Swainson. The former author asserts that the animals are 

 distinct from Melania, but the shells of several intermediate spe- 

 cies appear to indicate a connection, and if the soft parts present 

 a similar change, there w^ill be some difficulty in pointing out 

 the extent of the genus ; and indeed, to separate the Melanians 

 from the Cerithinse. If ' Pirena aurita' be placed in the former, 

 the distinctions between the two families, must be looked for in 

 the animal of Potamis. 



