212 INVERTEBRATA OF MASSACHUSETTS. 



Limn6a heter6stropha, SAY; Nicholson's Encyc., (Amer. ed.), Art. Conchology, 



pi. 1, f. 6. 

 Physa heterostropha, SAY ; Long's 2d Exped., Append., 264. LAM. ; An. sans 



Vert., viii. 402. 



" Shell sinistral, sub-ovate ; color pale-yellow, chestnut, or 

 blackish ; whorls four, the first large, the others very small, ter- 

 minating rather abruptly in an acute apex ; aperture large, some- 

 what oval, three fourths the length of the shell, or rather more ; 

 within of a pearly lustre, often blackish ; lip a little thickened on 

 the inside, and tinged with dull red." (SAY, in Nick. Encyc.) 

 Ordinary length about \ inch, breadth J inch, divergence 68. 

 My largest specimen is -^ inch by f inch. 



When the shell is fresh and perfectly clean, it is always of a 

 light greenish-yellow, and becomes a little more dusky with age. 

 The surface, under the magnifier, appears beautifully checkered 

 with minute, revolving, and longitudinal lines, which are also a 

 little waved. Sometimes there are one or more whitish, opake 

 bands, as if scratched by the mantle of the animal. The thicken- 

 ing of the lip is found only in old specimens, and in these also 

 there is a broad layer of pearly enamel reflected over the columella, 

 which has also a very prominent fold. 



The animal is olivaceous, surface very smooth and silky ; the 

 foot is kite-shaped, longer than the shell, terminating in an acute 

 point ; expansions each side of the mouth acutely angled ; ten- 

 tacula olivaceous above, light ferruginous beneath, long and thread- 

 like. The pointed lobes of the mantle are very conspicuous. 



The motions of the animal are very rapid, and it seems to 

 move with equal facility in an inverted posture, at the surface of the 

 water. 



The ova are excluded, enveloped in a gelatinous substance, 

 about twelve or fifteen in number, and of an egg-shaped form. 

 In about a fortnight they escape from the jelly, and move about 

 with great rapidity. In fact, they are seen in motion for some 

 time previous, apparently struggling to disengage themselves from 

 their nidus. 



This shell is everywhere to be found. Scarcely a brook or 

 pool is met with but some of these shells will be found in it. It 

 is more especially to be found in the running brooks. 



