126 BULIMULUS-ORTHOTOMIUM. 



has not diverged in any important particular from that common 

 stock of which Ncesiotus, Protoglyptus and Orthotomium are the 

 modern and now geographically individualized remnants. Among 

 the minor distinctions between these groups may be mentioned the 

 coloration ; Orthotomium never having spiral bands, such as occur 

 occasionally in Ncesiotus and Protoglyptus, and commonly in typical 

 Bulimulus. 



The species of Orthotomium (as here limited) have been widely 

 scattered throughout the complex maze of Bulimuloid subgenera by 

 all authors up to a very recent date. In 1893 Dall reduced the 

 chaos of Lower Californian subgenera to partial order; early in 

 1896 the writer, recognizing the cardinal value of apical sculpture, 

 outlined the system herein adopted ; and in a later work Dall 

 demonstrates the essential unity underlying the widely diverse con- 

 tours of the adult shells. 



Three " sections " then, may conveniently be recognized ; although 

 the differences, it will readily be understood, have no great signifi- 

 cance. 



I. Columella bearing a strong callous lamina within the last whorl, 



Section Sonorma, p. 155. 

 II. No callous lamina upon the columella. 



1. Shell ovate or oblong; aperture much over a third the 

 shell's length, generally one-half or more, 



Section Orthotomium s. s., p. 126. 



2. Shell subcylindrical ; aperture less than one-third the 

 length of shell, Section Plicolumna, p. 151. 



Section Orthotomium (C. &. F.) Pils. 



This section comprises two groups of species, those of central and 

 eastern Mexico, Texas, etc., and those of Lower California. In the 

 latter group the shell is frequently striated spirally and granular ; 

 it is never so in the eastern group. 



Key to groups. 



I. Lip not expanded or hardly so, often thickened within ; no 

 spiral striation. 



a. Ovate-conic or oblong forms of central and northeastern 

 Mexico and southcentral and southwestern United States, 



Group of B. alternatus, p. 127. 

 a'. Ovate-globose or oval species of Lower California, 



Group of B. sufflatus, p. 135. 



