346 MANUAL OF THE MOLLTJSCA. 



examining a large series of silicified specimens, and of figuring 

 a perfect sTiell, ■with its operculum in situ. It has more the 

 aspect of a bivalve, such as Requienia Lonsdalii (PL XYIII., 

 rig. 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 planorhis ; unless it is a case conversely 

 parallel to Atlanta, in which both shell and operculum have 

 dextral nuclei. The afl&nities 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 Euomphalus (p. 267) is not supported by 

 the eyidence of Sir W. Logan's specimens. 



CLASS III.— PTEEOPODA. 



This little group consists of animals whose entire life is 

 passed in the open sea, far away from any shelter, save what is 

 afforded by the floating gulf- weed, and whose organisation is 

 specially adapted to that sj^here 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 troj^ics, and no less in Arctic seas, where by their 

 mjrriads 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 

 Bmaller and feebler creatures (e.g. entomostraca). In Hgh 

 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 tho Continent ; in the older rocks they are unknown, unless 

 some comparatively gigantic forms {conularia and theca) havo 

 been rightly referred to this order. 



