4 THE NAUTILUS. 



II. Var. ANGiSTOMUS Hald., pi. I, figs. 4 and 5. 



Planorhis bicarinatus var. angtstoma Haldeman, Mon., p. 7 

 (1844). 



" Shell small, aperture campanulate, with the throat narrowed." 

 (Haldeman.) 



Haldeman did not figure this " variety," and his brief diagnosis 

 copied above gives neither the dimensions of his type nor the locality 

 from which it came. The type is not to be found in the collection 

 of the Philadelphia Academy, and is appai-ently lost. 



The claims of this form for varietal recognition are but little, if 

 any, stronger than those of var. unicarinatus. In nearly all mature 

 shells of bicarinatus there is a tendency, more or less dcTeloped, for 

 the lip to become everted, and there is no difficulty in selecting a 

 series showing all degrees of variation in this particular from the 

 same locality. It is only occasionally that all the specimens in the 

 colony are affected at the same time and to approximately the same 

 degree. But sometimes this feature is very persistent; thus in the 

 set from which the figure was selected not only are all the specimens 

 (38) decidedly campanulate, but all previous lips were also campanu- 

 late, many of tlie specimens having two and some even three, the 

 result being that the outline of tlie shell is in several instances greatly 

 distorted. All the specimens from this locality were unusually flat, 

 being very wide in proportion to their height. Similar specimens in 

 all respects were collected in Bawbeese Lake, Hillsdale Co., Michi^ 

 gan, one of which, having a diameter of 16.5 mm., is only 4 mm. in 

 height immediately in front ol the aperture, the latter being 8 mm. 

 in height. Some of these also are greatly distorted, showing appar- 

 ently the effects of an unfavorable environment. Shells from Pine 

 Island Lake, Kent Co., and Four Mile Lake, Chelsea, Mich., are 

 somewhat similar, but the peculiar form is less strongly developed. 



In many cases, no doubt, the sudden expansion of the lip is to be 

 accounted for by unusually favorable food conditions at the time 

 when the animal had really completed its normal growth ; the con- 

 sequent suddenly-acquired corpulency of the animal necessitating a 

 special, rapid and expansive growth of the shell to accommodate its 

 increased size. This seems to have been the case with the colony 

 from which Figure 1 was taken. Up to maturity the shell was a 

 typical aroostookensis, when suddenly the enormously expanded lip 

 was developed. In this case, too, the whole colony was apparently 



