98 LAND AND FRESH WATER MOLLUSKS 



The lamellation of Segmentina is composed of irregular undulate 

 ridges, radiating from the axis of the shell. In Planorbula there are 

 four dentiform lamellae on the outer and two (one quite small) on the 

 axial side of the throat, in a general way mostly turned in the direction 

 of the coll, and the earlier series are absorbed as the animal grows. 

 The position and shape of these lamellae are remarkably uniform in all 

 the species. In Haldemanina the lamellae are more elongate and com- 

 plex, requiring a diagram to define their relations, but on the whole 

 more like Planorbula than Segmentina. (See Binney, Land and Fw. 

 Sh. N. Am., n, p. 137, figs. 226-7, 1865.) 



Segmentina (Planorbula) armigera Say. 



Planorbis armigerus SAY, Journ. Acad. Nat. Sci. Phila., n, p. 164, 1818. 

 HALDEMAN, Mon. Limn., p. 30, pi. iv, figs. 11-13, 1844. GOULD, Inv. 

 Mass., p. 205, fig. 138, 1841. Type locality, Upper Missouri. 



Planorbis armiger BECK, Index, p. 123, 1838. 



Range. New England and the Middle States, south to Georgia, 

 westward to Nebraska, and northward to Great Slave Lake. 



Lake Winnipeg and the Saskatchewan River ! Egg Lake, Saskat- 

 chewan : Red Deer' Battle River; Great Playgreen Lake, Manitoba; 



Fort Ellice and Fort Pelly ; Ver- 

 milion Lake ; Moose Factory ; 

 Tames Bay ! Fort Resolution, Great 

 Slave Lake ! 



FIG. 76. Planar- FIG. 77. Teeth Shell biconcave, of five whorls, 

 bula armigera Say. of P. -wheatleyi polished, with an olivaceous peri- 

 *,nat. size;*, teeth Lea, for com- ostra surface sli hd 



magnified. panson. 



concave in the center, the suture 



deep but not channelled, upper surface of the whorls with an obscure 

 carination, the last part of the last whorl expanded and suddenly de- 

 flected downward, base with a steeply funicular umbilicus, exhibiting in 

 scalar fashion all the whorls, and bordered by an obtuse carina ; peri- 

 phery of the whorls median, rounded ; lip simple, hardly thickened, con- 

 tinued across the body by a thin callus ; aperture at an angle of 45 to 

 the vertical axis ; surface sculptured by fine lines of growth and obso- 

 lete microscopic, inconstant, spiral striation. Max. diam. 8.0; min. 

 diam. 6.5 ; height 3.0 mm. 



This common species extends well to the north, but has not yet been 

 reported from any part of the Pacific drainage, where it appears to be 

 replaced by a very similar species, P. declivis Tate, which however has 

 not yet been collected north of the Umpqua River, Oregon, specimens 

 from that locality and from Nicaragua being in the National Museum. 



