THE NAUTILUS. 17 



One peculiar form can, I think, be traced directly to the habitat 

 of the animal. In nearly every lot of western shells are to be found 

 a number of specimens, very narrow and elongated, with both of the 

 lateral sides decidedly convex and with the sides nearly parallel. 

 When placed on a flat surface the shell rests on the middle of the side 

 and the ends are elevated and arched, giving a trough-shaped appear- 

 ance to the shell, when placed apex downward. Now,/?<scs is a 

 dweller upon reeds and other aquatic vegetation. When it lives 

 on the flat side of a reed or leaf it grows normal in shape and the 

 peritreme touches the surface all the way around. But when it lives 

 on a round reed such as Scirpus lacustris, which is narrower than the 

 full grown shell, it adapts itself to its position and grows to fit the 

 reed, the ends following the convex surface of its support and the 

 sides lapping down around the reed itself. 



The dimensions of the specimens figured are as follows : 



Fig. 1. Length 5.5, width 4, alt. 1.25 mm. 



Fig. 4. Length 7.25 width 4.5, alt, 1.75 mm. 



Fig. 37. Length 8.25, width 4.5, alt. 3 mm. 



Variable within the limits above specified, nevertheless, A. fuscus 

 is a consistent and well defined species, which need not be confused 

 with any of its allies. It differs from A. kirklandi by its more de- 

 pressed and more regularly oval shape and more nearly central, more 

 obtuse, less prominent and less eccentric apex ; from A. diaphanus 

 by its elongated, oval shape and more obtuse apex and from A. ob- 

 scurus by its more depressed, less acute and more central apex and 

 straight posterior outline. 



Var. eugraptus Pilsbry (1896), PI. I., figs. 10-15. 



Typically slightly narrower and considerably higher than the 

 typical fuscus, but subject to great variation in this respect. Figures 

 10-12 from New Orleans and 13-15 from Reeds L., Kent Co., 

 Mich., represent the extremes. Surface with "very fine, somewhat 

 irregular radial striae, more distinct toward the periphery." 



Type : length 6, width 4, alt. 1.8 mm. 



Fig. 10. Length 7, width 4.75, alt. 1.8 mm. 



Fig. 13. Length 7.25, width 4.25, alt. 2.25 mm. 



II. ANCYLUS DIAPHANUS Hald. (1841). PI. II., figs. 13-18. 

 This is a well marked species and, in all the localities where the 

 typical form is found, seems to be very constant in its characters and 



