202 MANUAL OF THE MOLLUSCA. 



Fig. 106. Maclurea Logani, ( S alter) L. Silurian. Canada. 

 ? MACLUKEA, Lesueur. 



Named after Wm. Maclure, the first American geologist. 



Shell discoidal, few whirled, longitudinally grooved at the hack, and slightly 

 rugose with lines of growth; dextral side convex, deeply and narrowly 

 perforated ; left side flat, exposing the inner whirls ; operculum sinistrally 

 sub-spiral, solid, with two internal projections (t t] one of them beneath the 

 nucleus, very thick and rugose. 



Fossil, 5 sp. L. Silurian. N. America ; Scotland (Ayrshire, M c Coy). 



This singular shell abounds in the " Chazy " limestone of the U. States 

 and Canada ; sections of it may be seen even in the pavement of New 

 York; but specimens are very difficult to obtain. We are indebted to 

 W. E. Logan, Esq., Geological Surveyor of Canada, for the opportunity of 

 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 (PI. XVIII. 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 operculum 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 have dex- 

 tral nuclei. The affinities of Maclurea can only be determined by careful 

 examination and comparison with allied, but less abnormal forms, associated 

 with it in the oldest fossiliferous rocks ; its relation to Euomphalus (p. 145) 

 is not supported by the evidence of Mr. Logan's spetimens. 



CLASS III. PTEROPODA. 



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 organization 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 



