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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 Melanie, 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 ra ee parts of the stream. The Anculose, on 
the other hand, live attachec 
waters ; they are of s sdentary 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 I 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 F’érussac 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 Mined but merely sinuated a to 
the outline of the aperture, as described by Férussac 
“ Pirena aurita” is not congeneric with Pirena (atra) nor Me- 
lania, but must be placed in the family Cerithine ; next perhaps 
to Potamis. The type is distinguished from the typical Melanie 
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 will be some difficulty in pointing out 
the extent of the genus; and indeed, to separate the Melanians 
from the Cerithine:. If ‘ Pirena aurita’ be placed in the former, 
the distinctions between the two families, must be looked for in 
the animal of Potamis. 
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