122 thp: nautilus. 



distortion, as if a season of moderate food supply had been followed 

 by an aldermanic dinner or two, resulting in sudden and phenomenal 

 growth followed by a penitential })eriod of more temperate eating. 



These bulgings as before stated, seldom if ever exhibit any 

 regularity of occurrence in the Limnoeas ; they often do, however, in 

 the nearly related pond snails Plduorbis and Physa as may be seen 

 not infrequently in Phmorbis glabratus wherein they seem to occur 

 in somewhat orderly sequence. In the beautiful little Physa 

 (Costatella) costata described by my venerable friend Dr. Newcomb, 

 a form which inhabits Clear Lake, California — these bulgings are 

 numerous and regular and are arranged nearly equidistant and give 

 a postively sculptured aspect to the shell, in distinction from the 

 suggestion of pathologic eccentricity like the bulgings of the Limna^as 

 and many of the Planorbis. 



The above variation in strength or ])rominence of growth or incre- 

 mental lines, as they are usually called, is also common to many of 

 the Planorbes, notably in P. corpulentxis Say, from the West coast, 

 and is also frequently exhibited in examples of the curious and 

 interesting genus Cariaifex another West coast form from Eagle 

 Lake, California and elsewhere, and in the equally curious little 

 shells of the genus Pompholyx; that enterprising and intelligent 

 collector Henry Hemphill detected a pretty coslate variety at the 

 Dalles of the Columbia River, wherein the ribs are quite evenly 

 placed and suggestive of frequent variceal thickening as in some of 

 the Strophias. 



Many of the land shells of North America, as well as exotic 

 species, exhibit varying aspects or facies of incremental sculpture, 

 and many who read this will at once call to mind the difference, in 

 this res})ect, of examples of Mesodon of the same species from differ- 

 ent localities, also of Patula alternata, and the remarkable sculptural 

 diversity of the forms now included with Patula strigosa and its syn- 

 onyms. There are other groujis of the Helicidce which might be 

 included, but these are sufficient for the purposes of illustration and 

 among the Bulimulidoe we have no lack of instances, and the group 

 Strophia of the Pupidcp, which has led Mr. Maynard into a sort of 

 conchological quick-sand or "slough of despond," furnishes a fur- 

 ther appropriate illustration. 



Second. Inclusive of the first or above class of variation, we have 

 the dinted or malleated aspect of sculpture, as if the shell had 

 received a succession of light blows from a small hammer, blows 



