320 Till-; CHICAGO ACADEMY OF SCIEN( 



obese body whorl. Say himself identifies desidiosa from western New 

 York in Long's Expedition, II, p. 863, where he says, "Lymnaus de- 

 sidiosus nob. Falls of Niagara." Some of these specimens, as well as 

 others from Seneca Lake, agree fairly well with Binney"> figure 

 and also with Say's specimens. Binney's figure us is also unlike the 

 historical desidiosa, which rarely has such a uniformly oval aperture. 

 There are, however, occasional specimens of obrussa which compare 

 as favorably with Binney's figure 68 as do the specimens mentioned 

 above, and for this reason it seems best to refer it to obrussa with a 

 query. (Compare figures 10 and 11, plate XXVI, with Binney's figure 

 68.) Another significant fact is that when Say described obrussa, in 

 1825, he made no reference to desidiosa, which was described four 

 years earlier (1821). This fact, together with his comparison with 

 clodes, is to the author conclusive evidence of the distinction of these 

 two species. 



The history of desidiosa in the American monographs is interesting 

 and clearly indicates that since Say's time little attention has been given 

 to closely analyzing this species. In all of his references, Say distinctly 

 indicates a shell of the palustris type. Haldeman describes and figures 

 the form here distinguished as obrussa and not the true desidiosa. 

 (Compare his plate with Say's fig. 3.) Many of Haldeman's figures 

 are abnormal and do not represent obrussa as it is usually developed. 

 Tryon, in his continuation of Haldeman's work (p. lOi), states that 

 many of the figures on this plate are not desidiosa, but a form of 

 columella (macrostoma). In this statement Tryon is wrong and could 

 scarcely have seen Haldeman's specimens, for a recent examination 

 proved them all to be referable to obrussa (desidiosa of authors), 

 although, as stated above, several of the specimens are abnormal. The 

 writer has collected many specimens similar to those figured on Halde- 

 man's plate. Binney, in his Land and Fresh- Water Shells of North 

 America, part II, makes obrussa a synonym of desidiosa, thus showing 

 that he considered the latter the small, smooth form and not the true 

 desidiosa- of Say, and his figure 68 is questionable for this reason. In 

 Baker's Mollusca of the Chicago Area, obrussa is described and figured 

 as desidiosa. Recently, Dr. W. H. Dall, in his Alaska Mollusca (p. 73. 

 fig. 51), figures Say's obrussa under desidiosa, but also refers, in his 

 synonymy, to Binney's figure 68. The European monographs have 

 given figures referable to obrussa rather than to desidiosa. 



Amidst the uncertainties caused by the absence of Say's types we 

 must look for a shell which is closely allied to elodes, but is smaller, 

 with more convex whorls, and possesses 5 instead of 6 whorls. Such 



