The Nautilus. 



Vol. IV. MARCH, 1891. No. 11. 



NOTES ON THE SCULPTURE OF AMERICAN LIMN.5;AS, ETC. 



BY ROBT. E. C. STEARNS. 



In several instances parties who have sent specimens of Ameri- 

 can Limnseas to the U. S. National Museum for identification, have 

 called attention to the sculpture of the specimens, as if this charac- 

 ter was peculiar to the examples sent by them, and not previously 

 observed ; hence the following notes which may be of some use in 

 connection with the study of pond-snails, by beginners in this class 

 of molluscan forms. 



The extensive geographical series, and numerous examples in the 

 National Collection show the sculpture characters to be as follows, 

 and that all of the so-called species, of which without doubt there 

 are at least " twice too many," exhibit in a greater or less degree 

 when a large geographical series is brought together, with an ample 

 representation of individuals, the same aspects of sculpture variation. 



First. The longitudinal incremental or growth lines are often con- 

 spicuously developed in forms that generally are smooth or with 

 scarcely discernible growth lines, and occasionally individuals are 

 met with in which there is apparently a serial or alternating ar- 

 rangement of bands or zones of fine and coarse growth lines, suggestive 

 of a positive longitudinal sculpture, which we may suppose to be 

 due to a more vigorous growth at one season than another. Again 

 the larger whorls often exhibit a tumid or bulging elevation or prom- 

 inence which is rarely or seldom regular, usually irregular in occur- 

 rence, and not infrequently so conspicuous as to produce absolute 



