196 PARTULA, TAHITI. 



and the ground color of the shell varies from light horny- 

 yellow to rich brown. A snail of this type is fairly well rep- 

 resented in fig. 19, a dark-brown individual being drawn. 

 Ninety-seven per cent of the adult snails of type 18 and 91 

 per cent of type 19 are sinistral ; while of the young, 100 per 

 cent from type 18 and 90 per cent from type 19 are sinistral. 

 In the ease of the snails of type 19, twenty-six sinistral adults 

 gave forty-six sinistral and one dextral young ; while two dex- 

 tral adults gave four dextral and no sinistral young. Among 

 the snails of type 18, 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. 18 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 19, which are either unstriped 

 or with very faint whorl-stripes, gave forty-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 unstriped 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 per cent are sinistral. This condition appears, 

 therefore, to be stable from one generation to another." 

 (Mayer) . 



Maruapoo valley, on the western side of Tahiti about 

 7 miles south of Tipaemi valley. 



This form is said to constitute 99 per cent of the snail 

 fauna of the valley, P. hyalina forming the remainder. Fig- 

 ures and description from Dr. Mayer. The name sinistralis 

 being preoccupied, I have substituted lava. 



15. P. FILOSA Pfeiffer. PL 27, figs. 4, 5. 



Shell perforate, conic-ovate, solid, sculptured with close 

 impressed spiral lines, hardly shining ; chestnut-colored, orna- 

 mented with ashen hair-lines. Spire conic, rather obtuse. 

 Whorls 5, flattened, the last as long as the spire, more con- 

 vex, columella slightly plicate above. Aperture a little ob- 



