346 MANUAL OF THE MOLLUSCA. 
examining a large series of silicified specimens, and of figuring 
a perfect shell, with its operculum in situ. It has more the 
aspect of a bivalve, such as Requienia Lonsdalii (Pl. XVIIL., 
Fig. 12) than of a spiral univalve, but has no hinge. Many of 
the specimens are overgrown with a zoophyte, generally on the 
convex side only, rarely on both sides. 
The Maclurea has been described as sinistral ; but its oper- 
culum is that of a dextral shell; so that the spire must be 
regarded as deeply sunk and the umbilicus expanded, as in 
certain species of planorbis; unless it is a case conversely 
parallel to Atlanta, in which both shell and operculum haye 
dextral nuclei. The affinities of Maclurea can only be deter- 
mined by careful examination and comparison with allied, but 
less abnormal forms, associated with it in the oldest fossiliferous 
rocks; its relation to Huomphalus (p. 267) is not supported by 
the evidence of Sir W. Logan’s specimens. 
CLASS III.—PTEROPODA. 
Tuts little group consists of animals whose entire life is 
passed in the open sea, far away from any shelter, saye what is 
afforded by the floating gulf-weed, and whose organisation is 
specially adapted to that sphere of existence. In appearance 
and habits they strikingly resemble the fry of the ordinary sea- 
snails, swimming like them by the vigorous flapping of a pair 
of fins. To the naturalist ashore they are almost unknown; 
but the voyager on the great ocean meets with them where 
there is little else to arrest his attention, and marvels at their 
delicate forms and almost incredible numbers. They swarm 
in the tropics, and no less in Arctic seas, where by their 
myriads the water is discoloured for leagues. (Scoresby.) They 
are seen swimming at the surface in the heat of the day, as well 
as in the cool of the evening. Some of the larger kinds have 
prehensile tentacles, and their mouths armed with lingual teeth, 
so that, fragile as they are, they probably feed upon still 
smaller and feebler creatures (e.g. entomostraca). In high 
latitudes they are the principal food of the whale, and of many | 
sea-birds. Their shells are rarely drifted on shore, but abound 
in the fine sediment brought up by the dredge from great 
depths. A few species occur in the tertiary strata of England 
and the Continent; in the older rocks they are unknown, unless 
some comparatively gigantic forms (conularia and theca) haya 
been rightly referred to this order. 
