324 MACLUREID^ — HALIOTID^. 



Shell thin, symmetrical, horn-shaped or discoidal, with whorls 

 more or less separate, keeled and sculptured. 



CYRTONELLA, Hall, 1819. Shell ovoid, trumpet-shaped; volu- 

 tions one or more in the same plane ; apex minute, making about 

 a single turn, and rapidly expanding beyond, peristome entire ; 

 dorsum angular or subcarinate ; surface sculptured. G. mitella, 

 Hall. Devon. ; N. Y. 



Family MACLUREID^. 



Maclurea, Lesueur. 



Etym. — I^amed after William Maclure, the first American 

 geologist. 



Distr. — Fossil, 12 sp. Palaeozoic ; North America, Scotland 

 (Ayrshire, M'Coy). M. Logani, Salter Ixxxii, 8, 9). M. magna, 

 Lesueur (Ixv, 10). 



Shell discoidal, few-whorled, longitudinally grooved at the 

 back, and slightly rugose with lines of growth ; dextral side 

 convex, deeplj' and narrowly perforated ; left side flat, exposing 

 the inner whorls. Operculum sinistrally subspiral, solid, with 

 two internal projections, one of them beneath the nucleus, very 

 thick and rugose. 



This singular shell abounds in the " Chazy " limestone of the 

 United States and Canada ; sections of it ma}'^ be seen even in 

 the pavement of New York. "We are indebted to Sir W. E. 

 Logan, of the Geological Survey, 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, than of a spiral 

 univalve, but has no hinge. Many of the specimens are over- 

 grown 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 cer- 

 tain species of Planorbis ; unless it is a case conversely parallel 

 to Atlanta, in which both shell and operculum have dextral 

 nuclei. The aflSnities 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 rela- 

 tion to Euomphalus (p. 218) is not supported by the evidence of 

 Sir W. Logan's specimens." — Woodward. 



Family HALIOTID^. 



Shell spiral, ear-shaped, with a greatly expanded, flattened 

 body-whorl, and large basal aperture ; dorsally perforated in a 



