Maryland Geological Survey 373 



margin of the inner side; surface of adult or medium-sized specimens 

 nearly smooth, or having very obscure lines of growth, crossed by faint 

 traces of longitudinal strise ; on young individuals, or the inner volutions 

 of larger ones, these lines are quite distinct in both directions, and form a 

 very neat, concellated style of ornamentation; internal casts sometimes 

 showing a slender longitudinal line on the center of the periphery. 



" The proportions are shown by the following measurements of a young 

 individual: Length 1.84 in., breadth of aperture 1.7 in., diameter of 

 aperture in the direction of the length or greater diameter of the shell 

 0.72 in. Some imperfect adult individuals before me, too much broken to 

 afford exact measurements, were evidently as much as three times the 

 linear dimensions of that from which the foregoing measurements were 

 taken. 



" This common species has been wrongly identified with several foreign 

 forms. D'Orbigny, in his Prodr. de Paleont., expressed the opinion that 

 his own N. Icevigatus, published in 1846 (not his N. laevigatus, 1840) is 

 synonymous with it; also the Indian N. sphcericus and N. orbignyanus 

 Forbes, and a Chilean form referred by Professor Forbes to N. Icevigatus. 

 Mr. Blanford, however, considers both of the Indian shells merely 

 varieties of N. bouchardianus d'Orbigny, and entirely distinct from 

 N. dekayi Morton. I have not the necessary specimens at hand to express 

 any decided opinion in regard to the Indian shells figured by Mr. Blanford 

 all belonging to the one species of N. bouchardianus; but I can fully concur 

 with him in the opinion that they are certainly distinct from N. dekayi 

 Morton. The latter differs, as stated by Mr. Blanford, in having its 

 umbilicus always filled with a solid shelly kind of columella, formed by 

 the thickening of the lip at its connection with the body of the shell on 

 each side instead of being perforated. N. dekayi also has its aperture 

 constantly more transverse, and its siphuncle always nearer the inner 

 side, as may be seen by our fig. la, pi. xxvii, which represents very 

 nearly the typical form of the species, as I know from a direct comparison 

 with Dr. Morton's type-specimen, now in the Museum of the Academy of 

 Natural Sciences at Philadelphia, from which type-specimen the fore- 

 going outline-cut showing the position of the siphuncle was drawn. 



