3 6 4 
MOLLUSCS. 
and in the Mediterranean. It is reddish brown, with irregular and variable 
greyish blotches and spots. It occurs in numbers on the Portuguese coast, 
and in stormy weather is sometimes cast upon the shores in such quantities as to 
be the cause of epidemics, and almost to render it worth while to extract the purple 
for economic purposes. 
Of another family ( Oxynoidce ), we may take as an example Lobiger, which 
includes elongate molluscs provided with a thin transparent shell, and having on 
each side of the body two long parapodia, used as fins in swimming. The animal 
has the power of casting off these lobes, and probably the posterior end of the 
foot can also be spontaneously detached. Four species have been described from 
the Mediterranean, Ceylon, Guadeloupe, and the Society Islands. They are 
separable on account of certain differences in the soft-parts, but in a conchological 
point of view they appear to be indistinguishable. In the family PleurobrancJiiclce, 
the typical genus Pleurobranchus contains a 
number of species from many parts of the 
world, two of which occur on the British 
coast. The one illustrated (P. peroni ) is a 
native of tropical seas, and was originally 
obtained from the Mauritius. The body is 
convex, ovate, with the mantle extending 
over the back, and having a free margin at 
the sides. Beneath this, on the right side, is 
situated the large branchial plume. The 
head is furnished with both labial and upper 
tentacles, and at the inner base of the latter 
are situated the eyes. A thin, membranous, 
flat shell is concealed beneath the mantle, 
over the back. P. ( oscanias ) membranaceus and P. plumula have both been 
found on various parts of the British coast. Both range as far as the Mediter¬ 
ranean. The spawn of P. plumula is deposited in ribbon-like and spirally arranged 
masses,^resembling those of Doris. 
The animal of the genus Umbrella, which typifies another family, is very 
large, and carries upon its back a flat, circular, external shell, recalling the form 
of an umbrella of the Chinese pattern. The gill, as in Pleurobranchus, is on 
the right side, beneath the mantle, and protected by the shell. This unsightly 
creature is remarkable for the enormous development of the foot, which extends 
everywhere beyond the margin of the shell, and practically encloses the head in 
front. One species occurs in the Mediterranean, and three or four others are met 
with in the Indian and Pacific Oceans. The shell of U. indica is sometimes fully 
5 inches in diameter. 
The families Siphonariidce, Gadiniidce, and Amphibolidce, already mentioned, 
constituted, until recently, a suborder of Pulmonata, to which Gray gave the name 
Thalassophila. According to Haller, however, Siphonaria and Gadinia should be 
regarded as modified Opisthobranchiates, and placed next to the Umbrellidce ; but 
their systematic position cannot be considered definitely settled. The Siphonariidce 
look like ordinary limpets, and attach themselves to rocks in the same way at 
Pleurobranchus peroni (nat. size). 
