THE NAUTILUS. 31 



able in shape, ranging from planorboid with the apex depressed 

 below the level of the body whorl to specimens with the apex quite 

 as much elevated as in the ordinary specimens of lewisii. In sculp- 

 ture it varies from almost perfectly smooth to a striation nearly as 

 strong as that of typical lewisii. Indeed the more elevated and 

 strongly striated examples standing alone would unhesitatingly be re- 

 ferred to that species. 



Dr. Dall, who kindly compared specimens with his V~ lewisii heli- 

 coidea writes : " The figured specimen (helicoidea} should have (to 

 be adult) about one-half more whorl than it has in the figure. Some 

 of yours, if the same view be taken, are almost exactly like the speci- 

 men figured. But, on comparison of adults, it is evident that the 

 lewisii helicoidea has a smaller umbilicus ; and, taken in profile, is less 

 flattened. Your specimens, too, are on the whole smoother and 

 more polished. The value of these differences remains to be deter- 

 mined." 



The wider umbilicus in the Lake Michigan shells is undoubtedly 

 correlated with the more depressed shape. The specimens collected 

 by Lewis in the Little Lakes, N. Y., are indistinguishable from those 

 from Lake Michigan. The Lake Erie and Lake Ontario specimens 

 are uniformly smaller, but otherwise substantially the same. 



The difficulty in finding any tangible distinction between the more 

 elevated, substriated individuals of this form and the obsoletely 

 striated examples of V- lewisii is similar to that experienced in at- 

 tempting to separate V- tricarinata simplex from the smooth or obso- 

 letely striated forms of V. sincera. And this naturally raises a query 

 as to whether there are really more than two polymorphic species 

 differing constantly in shape, but both varying in sculpture, all the 

 way from tricarinate to smooth, and from smooth to ribbed-striate. 

 Both series are complete and parallelism is absolutely exact. 



Group of Group of 



V. tricarinata. V. bicarinata. 



Tricarinate, V- tricarinata. V- bicarinata nor- 



malis. 



Bicarinate, Y. tricarinata confusa. V- bicarinata. 



Unicarinate, V. tricarinata unicarinata. V- bicarinata con- 



nectans. 



Smooth, Y> tricarinata simplex. V. bicarinata per- 



depressa. 



