22 Calvin Goodkich 



At Black Bluff on the Coosa River, a short distance below Rome, Mr. 

 Smith's largest speci^nen of doitiiiei measures 17 x 10 1-2 mm., at Cedar 

 Bluff in Cherokee County, Alabama, the size has increased to 18 1-2 x 14 1-2 

 mm. This last is the largest lot collected by J\Ir. Smith and it seems to be 

 indicated that here the species is most flourishing. In Terrapin Creek, the 

 same county, conditions somev^hat identical with those in Georgia again 

 obtain. The average size of the adult shells is 14 x 9 1-2 mm., the maxi- 

 mum, 16 X II mm. Changes below this point are not great, yet are notice- 

 able. At Fitz's Feri-y, the largest downiei measures 16 1-2 x 11 1-2 mm., 

 at Maple Grove, 14 x 10; near Slackland, Etowah County, 15 1-2 x 9 1-2; 

 Lonigan Shoals, 14 x 11 ; Fomby Shoals, 15 x 11 ; Leoto Shoals, 16 x 11 ; 

 Riverside, St. Clair County, 15 x 10 1-2. After Cedar Bluff, it would ap- 

 pear, the species tends to run out numerically, and this is accompanied by 

 physical alterations. 



The shells of any one colony of downlei show little of the extreme varia- 

 bility which marks the Pleuroceridae in general, maintaining about the same 

 proportions of altitude to diameter from young to adult. 



In typical specimens, the surface of the body whorl is covered with folds 

 from suture to base. Below the peripher}^ these ordinarily proceed regularly 

 to the lip and are only occasionally wavy or wrinkled. Near the suture the 

 folds are very much waved where they "pass over" the plicae. In the case of 

 the up-river shells, the folds are narrow, sharply cut, but as one examines 

 the shells proceeding downstream one notices that the folds become broad, 

 smooth upon their surface, so nearly upon a plane that in poor light the 

 shell to the eye seems smooth. At Coosa, Georgia, specimens appear which 

 are very nearly smooth near the base and in the Cedar Bluff lots are several 

 individuals of this character. Of thirty shells taken at Leoto Shoals, 

 seven only have the typical folds; in the instance of twenty the folds are 

 obsolete or nearly so below the periphery and three are entirely without 

 folds. It is as if a characteristic wholly absent or rare in up-river forms 

 had become the normal habit on Leoto Shoals. In almost every lot there 

 are a few shells whose folds are broken into beads where the plicae are 

 crossed. The "obscure, transverse revolving striae" of Lea's description are 

 usually not continuous, being absent or faint upon the tops of the folds. Lea 

 lays as much emphasis upon his shells being plicate as upon their being fold- 

 ed. But the plicae are not a constant character. They differ in size and 

 in number. They are not always present. . Study suggests that the plicae 

 are caused by the massing of shell material at the top of the columellae 

 during the rest periods and that individuals are governed by no fixed 

 natural law as to the amount of the accumulation. \\'hen new growth be- 

 gins, the secretion is in smaller quantity, and thus an effect is given of 

 minature hills and valleys, and in as much as the "valleys" are often filled 

 with dark coloring matter the plicae in such cases stand out conspicuously. 



The color bands are in the texture of the shell as well as in the epider- 

 mis, being defined most clearly within the aperture. These bands consist 

 of interrupted deposits of pigment, customarily laid within the folds though 

 often "spilling over" into the creases between the folds. Sometimes the 



