THE NAUTILUS. i 



V. Var. AROOSTOOKENSis Pilsbry, pi. I, figs. 1 and 2. 

 Planorbis bicarinatus aroostookensis Pilsbry, Naut, VIII, p. 115, 



1875. 



Shell having the spire and umbilicus very deep, the latter I'unnel- 

 shaped, as in typical bicarinatus, but both upper and lower keels 

 entirely obsolete or rounded off on the last whorl, which has the 

 aspect of P. trivolvis. Surface minutely striated spirally, as in P. 

 bicarinatus. Aperture less angular and less produced below than in 

 bicarinatus, in consequence of the rounding of the whorls. Diameter 

 15 mm., alt. at aperture 7.5 mm. (Pilsbry). 



Type locality, East Branch of Salmon Brook, Woodland, Aroos- 

 took Co., Maine (Pils.). Also Collins' Pond, Caribou River, Cari- 

 bou, Aroostook Co., Me.; Charlevoix; Perch Lake, Kent Co.; Bes- 

 semer, Gogebec Co.; Fallams, Menominee Co., and Ispheming, Mich. 

 (Walker). 



Figure 2 represents the typical form of this variety, and is drawn 

 from a topotype collected by Mr. O. O. !Xylander. Figure 1 is the 

 campanulate form, already referred to, which bears the same relation 

 to typical aroostookensis that angistonius does to typical bicarinatus. 



The Michigan specimens, especially those from the Upper Penin- 

 sula, are much smaller than the typical form, and in some examples 

 there is a tendency to angulation on the whorls, but the large 

 majority of the specimens are quite characteristic in the rounded, 

 ecarinate form of the last whorl. 



VI. Var. STRiATUs Baker. 



Planorbis bicarinatus striatus Baker, Xaut. XV, p. 120 (1902); 

 Planorbis bicarinatus striatus Baker, Trans. Acad. Sci., St. Louis, 

 ivi, p. 9, pi. i, fig. 11 (1906). 



In this form the microscopic, revolving sculpture commonly but 

 not invariably present in the typical form is greatly intensified, and 

 in the typical expression of the variety is quite conspicuous to the 

 naked eye. 



In the original description this sculpture is stated to consis' of 

 " raised spiral lines," while in the second paper quoted above it is 

 Baid that " in some specimens the spiral lines are deeply incised." 

 This incongruity is an apparent one only, and depends on whether 

 the elevated or the depressed portions of the surface are wider or 

 more conspicuous. Starting with the typical form, in which the 

 revolving sculpture is either entirely wanting or very minute, it is 



