BULIMULUS-SONORINA. 155 



axis appears to be destitute of any lamellse, plications or projections 

 of any kind. In fact, the creature, so far as the shell is concerned, 

 is a very attenuated Leptobyrsus with its gyrate axis continued into 

 the adult state, whereas in the ordinary Leptobyrsus the gyration 

 ceased in time for the pillar of the completed shell to appear nor- 

 mally straight and the axis impervious. In a perfectly adult B.ramen- 

 tosus the margins of the aperture are expanded and slightly thick- 

 ened, but not reflected. The diameter of the hollow axis varies in 

 different specimens" (Dall). 



Section Sonorina Pilsbry, 1896. 



Leptobyrsus CROSSE & FISCHER, Moll. Terr, et Fluv. Mex., i, p. 

 475, 1874. DALL Proc. U. S. Nat. Mus., xvi, 1893, p. 642. Not 

 Leptobyrsa Stal. Sonorina PILS., Nautilus, ix, p, 114, 1896. 



Bulimuli with the nepionic whorls vertically costulate, the colum- 

 ella bearing within the last whorl a strong, more or less spirally 

 twisted callous lamina. Type B. spirifer Gabb. Distribution, 

 Lower California and islands in the Gulf of California. 



That all the Bulimuli of Lower California are more closely allied 

 genetically than any of them are to the species of Orthotomium in- 

 habiting eastern Mexico, must be acknowledged to be almost demon- 

 strated. Keeping this fact in view, it is also clear that by the ex- 

 aggeration or acceleration of certain features of growth in a portion 

 of the species, the Peninsular stock has been differentiated into three 

 groups, superficially very distinct. Sonorina, by developing an in- 

 ternal callous lamina upon the columella, and Plicolumna by its 

 lengthened Cylindrella-like growth and emphatic apical keel, have 

 diverged from the normal Orthotomium type. 



The genesis of the several subordinate groups of Orthotomium, if 

 the views herein expressed be correct, is exhibited by the following 

 diagram : 



Probable phylogeny. Taxonomy. 



Original f Eastern Branch-Group of B. alternate j Orthotomium 8i s . 

 stock of ( Group of B. montezuma J 



Orthotomium ( Western Branch -I Group of B. ramentosus Plicolumna. 



(. Group of B. spirifer Sonorina. 



The name Leptobyrsus was changed to Sonorina on account of a 

 supposed preoccupation of that term by Leptobyrsa in insects. 

 This is in accordance with the usage of most working concholo- 

 gists in England at the present time, and with that of the American 



