INSTITUTE OF SCIENCE, PHILADELPHIA. 21 



I should add that the last whorl increases in size less rapidly than in P. 

 Dicryi and forms a somewhat smaller proportion of the surface, viewed from 

 above. 



Planorbis Disstoni n. s. 

 Plate 10, figures 2, 2 a. 



Shell large, thin, carinated above, high, with five and a half whorls, of 

 which two and a half to three are visible on the base ; sculpture of the usual 

 transverse elevated lines in harmony with the lines of growth ; upper surface 

 of whorls carinated to or nearly to the aperture, slope from the carina to the 

 suture not very steep, nuclear whorls rather deeply sunken ; periphery rather 

 evenly rounded ; base subangular, central perforation deep ; suture well 

 marked, not very deep ; aperture in perfectly normal examples, extending 

 below the plane of the base and above the plane of the upper side, enlarged, 

 hardly campanulate ; callus on the body and outer lip somewhat thickened; 

 last whorl rather rapidly increasing in height compared with the last species. 

 Max. diam. 22.0; min. diam. 17.0; alt. 10.5 mm. 



This shell differs from all the trivolvis group, to which it belongs, in having 

 one more whorl, and in having on the base one more visible whorl. It is 

 variable and frequently distorted, so that perfectly normal specimens are rather 

 the exception. It differs from P. Couanti (which is more related to glabratus 

 and Diiryi than to trivolvis), in its greater proportional height, its more rapidly 

 expanding whorls, its carination on the spire and its somewhat sharper sculp- 

 ture. Some of the specimens vary in the direction of P. Conanti in a way 

 which suggests hybridization, but when full-grown and with a normal devel- 

 opment they are immediately separable from one another by the greater 

 height of P. Disstoni. This species is rather more abundant in the marls than 

 P. Conanti and less so in the silicified mud of the Planorbis bed, above them. 

 It is named in honor of Mr. Hamilton Disston, president of the Okeechobee 

 Land and Canal Co., to whom we are indebted for assistance in the explora- 

 tion of the Caloosahatchie beds, through the facilities possessed by the com- 

 pany in that region. 



Planorbis exacutus Say. 

 P. exacutus Say, Binney, L. & F. W. Sh. N. Am., II. p. 126, f. 210, 1865. 



Caloosahatchie beds, rather rare ; Dall, Willcox. 



This species still lives in the region and extends in the United States over 

 the greater part of the Atlantic watershed. 



Genus ISIDORA Ehrenberg. 

 Subgenus Ameria H. & A. Adams. 

 Thomsonia Ancey, 1888, not of Signoret, 1879 {Hemiptera). 



