134 THE NAUTILUS. 



spite of the fact that the mountains of the smaller island are 

 plainly visible from the vicinity of Caniyal. Between Porto 

 Santo and Madeira is deep sea, and it is evident that the islands 

 could not have been united within the life-time of existing 

 species of snails. Nevertheless, among the scant half-dozen 

 forms common to the Caniyal and Porto Santo deposits is P. 

 bowditchiana, one which would seem among the least likely to 

 be accidentally transported across the sea. It is a heavy white 

 shell, resembling in a general way the well-known Leucochroa 

 candidissima of the Mediterranean basin. The vast quantities 

 at Canigal suggest at first a former climate very different from 

 that of today, but the snail was doubtless adapted to arid or 

 semiarid conditions, such as prevail now at the eastern end of 

 Madeira. The sandy wastes of the locality swarm with living 

 snails at the present time, but they are with inconspicuous ex- 

 ceptions Helix pisana, probably introduced long ago from Africa 

 by the Moors. The P. bowditchiana, however, would need more 

 cover than exists now, as they were nocturnal, hiding by day, 

 if we may judge by the habits of their living relatives. I have 

 carefully compared the Madeira and Porto Santo P. bowditchi- 

 ana, thinking that some differences might be found, but they 

 are unquestionably identical. There is, however, this differ- 

 ence ; the Porto Santo specimens show a much wider range of 

 variation. This may be taken to indicate that the species evol- 

 ved on Porto Santo, and the one or more examples which 

 somehow reached Madeira started a colony which remained 

 essentially true to the type represented by the immigrants. 

 Twenty P. bowditchiana from Porto Santo varied as follows in 

 dimensions, the first figure of each pair being the length, the 

 second the diameter at right angles to the axis, both in mm. 15. 

 18 (1), 15.20 (1), 16.20 (2), 17.19 (1), 18.21 (2), 20.22 (1), 

 21.22 (2), 21.23 (1), 22.22 (1), 22.24 (2), 23.23 (3), 24.27 

 (1), 25.24 (1), 25.25 (1). The shell having a length less than 

 18 mm. (obtained in the vicinity of the Fonte d'Areia) consti- 

 tute a distinct form or variety, which may be named reducta. 

 They are not only small, but differing from the related small 

 P. punctulata, they are broadened, with a strongly and evenly 

 arched outer lip, so that the whole shape is very like that of 



