Occasional Papers of the Museum of Zoology 27 



The structure of the operculum (fig. ii-8) is quite similar 

 to that of T. plicatula (fig. ii-9), hut the calcareous plate is 

 heavier and more extensive, and is less apt to disintegrate in 

 the dried specimens. The whorls of the operculum increase in 

 size very gradually, so that the outline is almost circular, and 

 the nucleus practically central. Its appearance is almost as 

 close to that of Annularia as to that of Tudora incgacheilos 

 (fig. ii-io). 



The general appearance of this species is closest to that of 

 T. aripcnsis (Guppy, 1864) from Trinidad. The latter has 

 more regularly spaced riblets and the shape of its operculum 

 is more or less intermediate between that of T. plicatula and 

 that of T. u'illiaiusoni. 



Si)ecimens measure : 



Height inside Diam. inside 



Altitude Alaj. Diam. of Aperture of Aperture 



Largest female 12.1 55 (6.6) 26 (3.2) 100 (3.2) 



Another female 10.6 57 (6.0) 25 i2.y) g6 (2.6) 



Male; type 7.2 67 (4.8) 33 (2.4) lOD (2.4) 



Tudora williamsoni secana, new subspecies 

 Eleven specimens; on rocks and bare clay banks of an arroya 

 in the Cerritos de Yumarito, near Ouebrada Seca (H, II, a, 

 28). With it occurred Brachypodella hanlcyana, and. in the 

 leaf humus nearby, Tudora plicatula. 



This subspecies dithers, from the typical form, mainly in its 

 larger size. In addition, it is usually heavier, with darker 

 groimd color and more obscure color pattern. The males of 

 this lowland form are of about the same size as the females 

 of the mountain one. Representative specimens measure : 



Height inside Diam. inside 



Altitude Maj. Diam. of Aperture of Aperture 



Female (type) 14.1 54 (7-6) 26 {^-l') 100 {2,.7) 



Male 10.7 62 (6.6) 25 {2.7) 100 (2.7) 



