364 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 Pleurobranchidce, 



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 

 Pleurobranchus peroni (nat. size). are situated the eyes. A thin, membranous, 



flat shell is concealed beneath the mantle, 

 over the back. P. (oscanias) membranaceus and P. plurnula 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 



