LYMN^EIDyE OF NORTH AMERICA. 347 



fiat-sided zvhorls, the attenuated, steeple-shaped spire, the shallow su- 

 tures and the long, narrow aperture. Many specimens have zebra-like 

 markings (the form named zebra by Try on) and in not a few indi- 

 viduals the inner lip is erect and causes the whole aperture to stand 

 out from the body-whorl. The axis has a distinct twist and some indi- 

 viduals have an almost gyrate axis. This fact has led some concholo- 

 gists to place it in the genus Acella. The axis is not typically gyrate, 

 a fact which may be verified by breaking open the whorls ; the species 

 is a typical Stagnicola. There is considerable variation in the shape 

 of the aperture ; in many specimens the twist, so characteristic in re- 

 flexa, is entirely absent, while in others, notably in old specimens, the 

 twist is markedly developed. The whorls also vary somewhat in ro- 

 tundity. 



Lea's type of exilis is a large shell, with flat-sided whorls and 

 with the aperture entire and separated from the body whorl, a patho- 

 logic condition frequently seen in the reflexa group. It was collected 

 near Cincinnati, Ohio. The figure of exilis in Binney is very poor 

 and does not well represent this species. Figures 7 and 9 on plate 

 XXXVII are good examples of exilis as shown by Lea's type specimen. 

 There is some variation in the width of the body whorl (1, 2, 7, 8, 

 pi. XXXVII), but all have the very flat-sided, oblique whorls and long 

 aperture, which will distinguish exilis from all related species. Its 

 relation to lanceata is indicated in the remarks under that species. 

 Tryon's zebra is a synonym of exilis, according to his types and pub- 

 lished figures, although he seemed to make it cover any zebra-marked 

 shell of the reflexa group, a fact borne out by his remarks in the 

 American Journal of Conchology, volume I, page 228, where reference 

 is made to Haldeman's figures of umbrosa and reflexa, to which the 

 true zebra is not especially related. Of Tryon's type lot of four speci- 

 mens, only one corresponds with his figure in the American Journal 

 of Conchology, plate 23, figure 4, the others being true exilis. A pe- 

 culiar, small form of exilis occurs near Des Moines, Iowa, which is 

 small and narrow, with a very heavy varix in the outer lip, showing 

 that the animal at some time sustained a long period of aestivation. 

 The body whorl is very much flattened. This form is frequently iden- 

 tified as kirtlandiana Lea. When perfect, exilis is one of the most 

 graceful and beautiful of the Lymnaeas. 



Dr. Dall (Alaska Moll., p. 72) believes that exilis should be 

 included in the Acella group with Haldeman's gracilis ( = haldemani) . 

 While there is a superficial resemblance to haldcmani in the flatness 

 of the whorls and in the general oblique and elongated shell, an exam- 



