16 THE NAUTILUS. 



setts, none of which is typical in size, does not show any considerable 

 variation in the contours of the shell. But in the west, where it is 

 an abundant species, there is considerable variation in this respect. 



In 1896 (Nautii.us, IX., p. 139), Dr. Pilsbry described a shell 

 similar in shape, though narrower and higher, with the surface 

 ornamented with " very fine, somewhat irregular, radial striae, more 

 distinct toward the periphery " as A. eugraptus. 



The large amount of material examined has forced me to the con- 

 clusion that eugraptus \s only a. ribbed ^ovm oi f us cus. In almost 

 every considerable number of specimens, all the variations can be 

 found from those with a smooth surface, through those with the sur- 

 face more or less radially rippled, to those with the fine ribs of 

 typical eugraptus. This variation in the sculpture is not confined to 

 the western specimens. In two sets of A. fiiscus from Winchester, 

 Mass., in different collections, which, so far as shape and contour is 

 concerned, are entirely typical, the surface varies from the typical 

 smooth fuscus to examples with as well developed ribs as the major- 

 ity of the western eugraptus. Nor are the western specimens of 

 eugraptus uniformly higher and narrower than the typical eastern 

 examples of fuscus. While, perhaps, they average higher than the 

 eastern specimens, they vary insensibly from the depressed form of 

 typical y?<5c<<5 to elevated specimens higher than the typical eugrap- 

 tus, so that I have not seen my way clear to separate the eastern 

 from the western form on any substantial difference in shape. 



Assuming the Massachusetts form to be typical fuscus, it may be 

 described as a depressed, oval or slightly obovate shell, with the left 

 side more arcuate than the right ; anterior and right slopes straight, 

 posterior and left slopes slightly convex ; apex very obtuse, not ris- 

 ing above the general outline of the shell, smooth, slightly behind 

 and to the right of the middle. Translucent horn-color, shining. 

 Surface with faint growth lines, otherwise smooth or with irregular 

 and discontinuous transverse ripples which tend to form irregular 

 radial riblets. 



From central New York to the west there appears to be a much 

 greater degree of variation. The shells tend to become narrower and 

 more elevated, and with a greater convexity to the left slope. But 

 throughout the peculiar, rounded, obtuse apex remains as a valuable 

 specific characteristic in differentiating it from A. kirklandi, dia- 

 phanus and obscurus. 



