112 CLASS III. GASTEROPODA. ORDER VI. PULMOBRANCHIATA. 
their respiration for a considerable time ; a striking example in the eco- 
nomy of Nature, ministering, as it were, to the existence of such as may 
remain in fens or marshes, where the water is occasionally dried up. 
During the winter season the Lymnzana have been observed to remain 
under water for several months ; and it has been supposed, that besides 
having a pulmonary cavity, they are provided with a true branchial appa- 
ratus; D’Orbigny, for example, an intelligent naturalist, who has had 
peculiar facilities of observing the nature and habits of the Mollusca, 
seems quite unwilling to admit of this suspension of the animal powers ; 
he believes that they have the faculty of absolutely separating from the 
water so much air as is necessary to preserve them in their hibernacle. 
‘En résumé, de ces deux faits, opposés en apparence, nous croyons pou- 
voir inférer que les animaux de cette série jouissent simultanément de la 
double faculté de respirer l’air en nature, au moyen d’une cavité pulmo- 
naire, et de séparer l’air de lV’eau a l’aide de branchies ; au moins est-ce 
la ce que leur genre de vie nous force d’admettre.” 
The Lymnzana may be described as having a rather thin, smooth shell, 
the aperture of which is generally large, with the margin simple, acute, 
and never reflected. We divide them into three genera, as follows : 
PLANORBIS. ANCYLUS. 
LyMN-A. 
PLANORBIS, Guettard. 
Testa discoidea, spira depressd, apice vix prominulo, anfractibus rotun- 
datis, utrinque conspicuis, ultimo interdum carinato ; apertura inte- 
gra, semilunari, marginibus disjunctis ; labro aut simplici, aut in- 
crassato, nunquam reflexo. 
The genus Planorbis, founded by Guettard, a skilful conchologist co- 
temporary with Adanson and Linnzeus, has been for the most part adopted 
