43 



far beyond the lower extremity ; umbilicus as wide as the penult 

 whorl. The following are the dimensions of a large specimen. 



Greatest breadth .075 inch ; least breadth .06 inch ; height .03 

 inch. 



Planorbis Mac.Nabianus. Shell discoidal and excessively flat- 

 tened, like P. vortex ; brownish horn color ; with microscopic 

 crowded transverse strite of growth ; spire a little more concave on 

 the right than on the left side ; apex conspicuous on both sides ; 

 whorls six, with a well impressed suture on both sides, planulate on 

 the left side and a little convex on the right side ; last whorl with 

 an angular periphery in the plane of the left side ; aperture very 

 oblique, the right half of the labrum being advanced very far and 

 abruptly beyond the left half, especially in old shells, in which the 

 advance is equal to twice the diameter of the aperture. 



Greatest breadth .27 inch ; least breadth .25 inch ; height of the 

 last whorl .04 inch. 



Planorbis Redfieldi. Shell discoidal ; horn color ; smooth, 

 with a few microscopic interrupted spiral strife ; spire more widely 

 and deeply concave on the left than on the right side ; apex visible 

 on both sides ; whorls nearly four, wider on the left side, with a 

 well impressed and margined suture on both sides, subplanulate on 

 the left, convex on the right ; last whorl spreading much through a 

 regular curve from the right to the left side, where it has a suban- 

 gular periphery, narrow on the left side ; labrum with its right half 

 advanced beyond its left by a space equal to the diameter of the 

 aperture. 



Greatest breadth .185 inch; least breadth .16 inch: height of 

 last whorl .065 inch. 



Planorbis Haldemani. Shell subdiscoidal ; brownish horn col- 

 or ; with very fine striae of growth, and several spiral series of 

 microscopic punctures ; spire very narrow ; apex deeply sunk on 

 both sides ; whorls five, rather convex, more convex on the right 

 side, with a well impressed suture margined on the right side ; last 

 whorl very wide on both sides and high, with a slightly angular 

 periphery on the left side ; aperture lunate, with the labrum most 

 advanced at one third of its length from the right extremity. 



Greatest breadth .14 inch; least breadth .125 inch; height of 

 last whorl .07 inch. 



Planorbis decipiens. Shell discoidal ; pale horn color ; with 

 fine stria) of growth and finer microscopic spiral striae ; spire wide,. 



