128 EXPEDITION OF THE "ALBATROSS," 1890-1900. 



provided with a well-developed tooth upon tlie columellar side, and there is 

 also a tooth upon the wall of the aperture. The surface of the shell is 

 smooth and slightly polished, and there are no deep longitudinal furrows. 

 About 57 per cent of these snails are well represented by Fig. 14. The 

 ground-color is a light horny-yellow streaked longitudinally with darker 

 brown, in addition to which there are two dark-brown whorl-stripes and a 

 white whorl-stripe adjacent to the suture of the spire. In about 43 per cent 

 the dark-brown whorl-stripes are either absent or very faint, and the ground- 

 color of the shell varies from light horny-yellow to rich brown. A snail of 

 this type is fairly well represented in Fig. 15, a dark-brown individual 

 being drawn. Ninety-seven per cent of the adult snails of type 14 and 91 

 per cent of type 15 are sinistral; while of the young, 100 per cent from 

 type 14 and 90 per cent from type 15 are sinistral. In the case of the 

 snails of type 15, twenty-six sinistral adults gave forty-six sinistral and one 

 dextral young ; while two dextral adults gave four dextral and no sinistral 

 young. Among the snails of tvpe 14, only one of the dextral adults con- 

 tained young, and this one gave a single sinistral offspring. Twenty-six 

 whorl-striped adults of the type of Fig. 14 gave forty-two young, of which 

 76 per cent display whorl-stripes, while 24 per cent are plain colored. On 

 the other hand, twenty-eight adults of type 15, which are either unstriped 

 or with very faint whorl-stripes, gave fifty-one young, of which 61.5 per 

 cent are marked with whorl-stripes and 38.5 per cent are plain colored. It 

 is very evident that the striped and luistriped forms intergrade. 



Of the one hundred adults found in the valley, 94 per cent are sinistral, 

 while of the eighty-eight young taken from them, 94.5 percent are sinistral. 

 This condition appears, therefore, to be stable from one generation to 

 another. 



It will be observed that P. nodosa is somewhat like P. filosa (Figs. 9 and 

 10) from Pirte valley ; it differs from the latter, however, in that there is 

 a well-developed tooth upon the columellar lip, and the surface of the shell 

 is polished and never furrowed, with dull-colored longitudinal grooves. 



