536 Annals of the Carnegie Museum. 



dark variety of alteniata is found, to which the above name may be 

 given. So far as seen it reaches its greatest development on Middle 

 Sister Island. 



Shell very dark, flame markings dark chocolate-brown and coalescing 

 into two almost solid bands at the periphery, frequently the bands 

 are hardly separated; below a band of the body-color with irregular 

 flames in the umbilical region. Fully adult shells frequently lose 

 most of the epidermis, but by transmitted light the two bands can 

 be seen at the periphery with the lighter band and color below. 

 Young, bright shells are generally very dark. Albinos are found on 

 all of the islands. Ribs strong and running w^ell down to the umbilicus, 

 regular on the upper whorls, but less so on the body-whorl, being very 

 irregular near the aperture. Body-whorl subcarinate, upper lip con- 

 siderably flattened, particularly on the shells from Middle Sister 

 Island, where it is frequently as much depressed as in Circinaria. 

 Lip thickened in old shells and the parietal callus very heavy, fre- 

 quently forming a strong ridge. Subscalariform specimens are not 

 uncommon. Whorls six to six and one-half in Middle Sister .=hells, 

 in those from other islands about five and one-half. 



There appears to be a greater tendency to albinism on Middle Sister 

 than on the other islands. Out of one hundred and seventy-six 

 collected by the writer five immature shells are of a uniform straw- 

 color and some of the adults, although badly weathered, seem by 

 transmitted light to be albinos. Others are straw-colored with in- 

 distinct flames. Twenty-five to thirty shells, or about i6 per cent, 

 are light-colored. 



In the umbilicus of several of the Middle Sister shells the empty 

 pupa-case of a fly, or wasp, w-as found filling the umbilicus com- 

 pletely and requiring a strong pull to detach it. 



Middle Sister Island. 102 fully adult shells measured: 



Diam. 21.81, Alt. 14.23 mm., Index, --65.25 average, 



70.00 largest, 

 74.40 smallest, 

 76.10 most elevated, 

 " 58.70 mcst depressed. 



Variation in diam. from 19.5 to 25.0 or 5.5 mm. 



" alt. " 12.5 " 17.5 " 5.0 " 



Eighty-eight shells varied in diam. from 20.5 to 23.5 or 3 mm. 

 Ninety " " " alt. " 13.0 " 16.0 " 3 " 



