TO GEOLOGY. 107 



small. The thickest part of the columella being above, 

 there is no reflection over the umbilicus. The suture is 

 very small. It differs from the gibbosa (herein described), 

 in having the whorls more flattened above, and in having 

 a small umbilicus. It differs also in the callus. 



A*, minor. Plate 4. Fig. 90. 



Description. Shell sub-elliptical, smooth ; substance of 

 the shell thin; spire produced and rounded at apex; sutures 

 impressed; columella thickened and reflected about the 

 middle of the umbilicus ; umbilicus large ; whorls four, 

 rounded ; mouth semi-lunate, about two thirds the length 

 of the shell. 



Length .3, Breadth .2, of an inch. 



Observations. This species somewhat resembles the 

 gibbosa but is very much smaller. It is remarkable 

 for its length and for the roundness of its whorls, in 

 which it has some resemblance to the Paludina. The 

 callous or thickened part of the columella is placed direct- 

 ly in the middle of the umbilicus, and in this specimen, 

 the only one I have seen, there are two or three angular 

 impressions on the lower part of it. I have not mention- 

 ed this character in the specific description, doubting if 

 it exists on all. 



JV. minima. Plate 4. Fig. 91. 



Description. Shell globose, smooth ; substance of the 

 shell thin ; spire flattened ; sutures impressed ; columella 



