1881.J 



NATURAL SCIENCES OF PHILADELPHIA. 



99 



then straighten it out, it woukl resemble, in minatiire, an acutely 

 elongated conical tube, in a general way like the following 

 figure : 



Fig. 14. 



X 



-^^ &s^ ' — - — N 



Fig. 



of which N represents the nucleus, A the aperture or mouth, and 

 ML a median line. Xow it will readil}^ be seen that such a tube, 

 if simply wound up, or made into a flat coil, and during the pro- 

 cess of winding, kept horizontalh^ and laterally in 

 plane with the central or median line which divides 

 the tube into equal parts, would, in an exceedingly 

 small shell, make it somewhat difficult to determine 

 which was the apical or the umbilical (that is the 

 upper or under) side of the shell, as the nucleus 

 and nuclear whorls in such a case would be equall}' 

 as perceptible on one side of the shell as on the 

 other, and the concavit}^ or depression of both 

 sides would be the same, bejng equal to one-half 

 of the diameter of the tube as seen at X. 



The Californiau species to which I have re- 

 ferred, instead o,f being represented b}^ an attenu- 

 ated tube like the preceding figure, which very 

 slowly increases in circumference from nucleus to 

 aperture, would if unwound, give us a more robust 

 form, a more rapidly enlarging, conical tube, like 

 this (fig. 15) : 



X being the nucleus, A the aperture, and ML 

 the median line. 



It will be seen that if this tube, commencing at 

 X the nucleus, was evenly coiled upon the median 

 line, the nucleus as in the first instance, though 

 very much more depressed, owing to the greater 

 diameter of the tube as seen at X, could be equally- 

 well seen on the two sides, the umbilical and apical 

 depressions being the same. 



When the line of coil is other than median, and 

 the greater portion of the tube or shell is below the 

 line of coil, as is the case with the species I find to 

 be sinistral, then of course the umbilicus is the more and the 



