OXYSTYLA. 103 



of its partial or complete interruption by dry periods. The influence 

 of these dry seasons is to check growth ; the dark pigment, which 

 during rapid growth in seasons of humidity is distributed in flames 

 or bands, is concentrated in. the narrow, slowly forming lip-streak; 

 finally growth ceases, the aperture is firmly sealed to the bark of a 

 shrub or tree by a thick epiphragm, and metabolism is reduced to a 

 minimum until the recurrence of wet weather abruptly initiates a 

 new period of active growth. 



Each varix corresponds to a light or imperfectly pigmented sig- 

 moid streak on the parietal callus, apparently caused by a temporary 

 deficiency of coloring matter upon the resumption of growth after a 

 resting period. Specimens having varices upon the latter portion of 

 the last whorl show these pale streaks on the callus. From the fore- 

 going, it will be obvious that local climatic conditions govern the 

 development of varices to a great extent ; and these markings are 

 therefore characteristic of the local races into which Oxystyla every- 

 where splits. 



The sculpture varies somewhat in different species, consisting of 

 spiral stride and oblique malleation. In 0. undata jamaicensis it is 

 almost or entirely obsolete ; in obducta it is very faint ; in princeps 

 sharp, irregular, the striae anastomosing or descending at short inter- 

 vals in a series of steps. In 0. bensoni the spiral lines are strongly 

 developed, decussating the longitudinal striae. 



None of the species, so far as now known, are common to South 

 America and Mexico or Central America ; the supposed Central 

 American obducta being a form of 0. princeps, and the Colombian 

 and Venezuelan shells referred by von Martens and Strebel to 0. 

 ferussaci and 0. tricinctus prove to belong to 0. maracaibensis. 



The Middle American species have been monographed by Fischer 

 and Crose, later by Strebel and Pfeffer (1882), whose work contains 

 a great amount or original and valuable information, and lastly by 

 Dr. E. von Martens (1893) who gives an excellent and succinct 

 account, marred only by an insufficient appreciation of the status of 

 the " melanocheilus " forms, and the inclusion of 0. obducta, a species 

 not occurring in North America. 



The species are all variable. Those of " Mexico and Central 

 America of the 0. undata-princ.eps group are nearly allied inter se, 

 and it may be questioned whether they do not run one into the other, 

 so much so as to be regarded as varieties of one and the same species. 



