lo Calvin Goodrich 



ities in shell characters. In A. fic.vuosa H. H. Smith there are like- 

 nesses of shell which link the species both with the picta and the taoiiata 

 group. There is likewise a dimorphism of opercula. 



In the picta group we have the largest opercula of the genus. They are 

 all easily recognizable and separable apart from their shells, the spiral lines 

 are usually well marked, there is a similarity of color and texture as there 

 is a similarity in the fact that all have loosely-coiled whorls. The opercula 

 of picta and fonnosa are alike in form, but the latter are large and the 

 spiral lines are more clearly defined. From the Conasauga down to Cedar 

 Bluff on the Coosa, the operculum of dozinei is more individual than it is 

 farther south where it tends to mirror the operculum of formosa. As in 

 the case of the shell, the operculum of modesta seems to be a degenerate 

 form of doiwiiei. The forcinaui operculum has the distinctive serrations 

 first noticed by Hartman. It is inclined in many instances to be definitely 

 triangular. Though in point of shell characters this species is closer to 

 formosa than is dozimiei, the operculum yet points to a more distant rela- 

 tionship. ]\Ir. Smith was at one time inclined to believe that his clipcata 

 deserA^ed erection as the type of a new genus upon the basis of the oper- 

 culum, which is large, thick-margined, generally very dark and with the 

 strong spiral lines near the center. It would seem that his opinion changed 

 in this matter. The operculum of clipeata is carried at the very opening 

 of the aperture, much as in the case of Bythinia tentaculata L. 



The opercula of the taeniata group have tightly-coiled spiral lines, us- 

 ually very indistinct. They are ovate to elliptical, ordinarily dark. In tae- 

 niata the opercula of young and half-grown specimens are usually elongate, 

 but as the animal grows older it widens the organ. Those of torrcfacta 

 though generally rougher resemble the opercula of taeniata just as the shells 

 are much alike. In coosaensis, the opercula are smaller, more regularly 

 formed and less variable than in taeniata. The normal operculum of grif- 

 fithiana is thick, dark, broader in proportion to altitude than in taeniata, 

 the growth lines strong. The ribbon-like operculum wfiich occurs in many 

 specimens is a variant developing with the juveniles and there are no in- 

 dications that it is brought about through accident or disease. The spiral 

 lines and the nucleus are absent. Growth proceeds from the left margin, 

 or that nearest the columella, along a straight line, turning outward slightly 

 at the edges. As the ordinary operculum resembles a leaf, so in this kind of 

 operculum the grain of a tree is suggested. Hartman (1871) was under 

 the impression that such opercula occured among individuals inhabiting still 

 water. Mr. Smith found it in griffithiana in numbers on the Coosa shoals. 

 In aldrichi we find opercula of the normal and the produced form common 

 to griffiiJiiana, though much smaller. In choceoloccocnsis, the operculum 

 is shapcfl like that of taeniata, but the spiral lines are more loosely coiled. 

 The operculum of hrcvispira is narrow, elliptical in the young, developing 

 the characteristic taeniata form not until well grown. It may be, described 

 as the antithesis of the operculum of clipcata, for it occupies only one-third 

 to one-half of the aperture, and when withdrawn must go far within the 

 shell. 



Jn the ampla group, the oj)ercula arc not easily to be distinguished from: 



