Anculosae of the Alabama River Drainage 9 



The Opercula 



The literature is singularly unhelpful regarding the opercula of the 

 Pleuroceridae. After the first superficial examination, the earlier natur- 

 alists took it for granted, or they seem to have, that little or no variation 

 existed in this character. Throughout the four hundred and more pages 

 of Tyron's Monograph of the Strepomatidae (1873) are just two items in 

 the text and two illustrations, one of them entirely unreferred to, which 

 give any indication of a recognition of differences. Dr. W. D. Hartman 

 (1871) mentioned the serrated operculum of Anculosa forcmani Lea and 

 the ribbon-like opercula of some specimens of Leptoxis riibiginosa Lea. 

 Yet he generalizes about the operculum as if he were convinced that these 

 two variants were but rare exceptions to a rule most firmly fixed. So keen 

 an observer as Lewis apparently felt that the opercula were not worth the 

 bother of examination though he was unorthodox toward the accepted 

 facts of his day relating to species and their relationships. Not until Pilsbry, 

 in describing Goniohasis comalensis (1906), pointed out a distinctive form 

 of operculum did anyone hint that the organ might vary sufficiently to war- 

 rant intensive study. 



From the beginning of his work in Alabama, Mr. Smith made a point 

 of observing opercula. From his correspondence one gathers the informa- 

 tion that he found them exceedingly helpful in separating puzzling forms 

 of the family. In one instance, where the nodose Goniobases (Eurycaelon) 

 and nodose Gyrotomae appear together in the Coosa River, the operculum 

 constituted a definite demarkation of generic character more conspicious 

 than a rudimentary Gyrotomic fissure. He had other experiences of the 

 sort among species of Goniobasis in the Cahaba River. The two Anculosae, 

 formosa and foremani, run very closely together in certain localities. The 

 opercula permit immediate separation without uncertainty. 



On December 6, 1905, Mr. Smith wrote to Dr. Walker: "Goniohasis 

 shozvalterii Lea, as you know, has the operculum greatly prolonged. The 

 species is common on some parts of Weduska, Peckerwood and Fort Will- 

 iam shoals, and I noticed that when the snail has its foot out, the operculum 

 curls over the body, lying on it very neatly. I have found the operculum 

 of great service in separating species of Goniobasis, but it is necessary tc 

 use great care. No part of the mollusk is so subject to accidents and dis- 

 tortion as is the operculum. In fact I am practically sure some Pleuro- 

 ceridae as well as Campeloma and Tulotoma may lose the operculum al- 

 together and form another. I have found living and apparently healthy 

 specimens without operculum, and I have found others with a tiny and al- 

 most transparent new-formed operculum in place of the normal large one.' 



The operculum sharply differentiates the species of the picta group frorr 

 one another and also from all the other groups. This is true as well in 

 regard to the line between A. ligata and other Anculosae. In the compacta 

 group, the line is cleanly drawn as against other groups, but probably only 

 the student of the genus would readily distinguish the variations of the 

 opercula of the three species belonging to it. There is, on the other hand, 

 a great deal of similarity among the opercula of the taeniata, ampla and 

 shozvalterii groups, and this supports to some extent certain general similar- 



