Occasional Papers of the Museinn of Zoology 5 



A. Streptostyla irrigua irrigua (Shuttleworth). Typical form. 



Spiraxis (Streptostyla) irrigua Shuttleworth (1852) ; 



Streptostyla i. C. and F. (1870) and von Mart. ( 1891). 



Streptostyla cingulata Crosse and Fischer (1868, 1870). 



The typical forms are those with a stronger tendency toward 



impressed lines and costulae near the suture; cingulata represents the 



extreme of this phase. 



B. Streptostyla irrigua shuttleworthi (Pfeiffer). 



Spiraxis shuttleiuorthi Pfr. (1856). 



Streptostyla sallei Crosse and Fischer (1868, 1870) ; Stre- 

 bel (1878), etc. 

 These are the larger shells which resemble, to a certain degree, 

 Streptostyla lattrei (Pfr.). From the original description, I certainly 

 agree with Strebel's (1878, page 51) observation, that true shuttle- 

 zvorthi of Pfeiffer seems closer to sallei and edimrdsiana C. and F., 

 and is not what is ordinarily known by that name. 



C. Streptostyla irrigua cdwardsiana Crosse and Fischer (1868, 



1870), and others. 

 This form is very close to the preceding, but is somewhat more 

 attenuate. 



D. Streptostyla irrigua .sinnilis Strebel. 



Streptostyla shuttleworthi C. and F. (1870), Strebel (1878, 

 p. 18), von Martens (1891), etc. 



Streptostyla similis Strebel (1878). 

 As used here, this includes the smaller thin-shelled forms, usually 

 called typical shutttczvorthi, and also the shells with less prominent 

 spiral, but more prominent vertical, sculpture, which are Strebel's 

 typical similis. Strebel's shuttlezvorthi and his similis are not dif- 

 ferent geographical, or probably not even ecological, races, as he men- 

 tions specimens of both, obtained in the same lot (p. 19). As the 

 differential characters are very variable, I do not think that a new 

 name is necessary for the form usually called shuttkworthi. 



E. Streptostyla irrigua ventricosa von Martens (1891). 



S. shxittleworthi veiitricosa von Martens (1891). 

 A slightly stouter form of what is here called similis. 



This whole group of forms, all of them described from 

 around Cordova and Orizaba, may be nothing more than varia- 

 tions or ecological forms of a northern subspecies of S. lattrei 



