THE NAUTILUS. 77 



entiation of the marsupial expansion and its advance from a position 

 at the extreme posterior part of the shell to a median one almost 

 directly under the beaks. 



An arrangement of the species in this manner would result as fol- 

 lows : 



1. Marsupial expansion occupying the entire post-basal area : 

 T. perplexa, T. biemarginata, 



T. perplexa rangiana, T. capsaformit, 



T. sampsoni, T. florentina, 



T. propinqua. T. deviata. 



2. Marsupial expansion formed by an inflation of the posterior 

 ridge : 



A. Not extending below the basal line : 



T. triquetra, T. arcaeformis. 



B. Extending below the basal line : 



T. penita, T. compacta, T. metastriata. 



3. Marsupial expansion in front of the posterior ridge and more or 

 less separated from it : 



A. Scarcely differentiated from the posterior ridge : 



T. hayesiana, T. modicella, T. othcaloogaensis. 



B. Separated from the posterior ridge by a narrow sulcus, notch- 

 ing the post-basal margin : 



T. brevidens, T. sulcata, T. lenior. 



C. Separated from the posterior ridge by a wide emargination : 

 T. personata, T. lewisii, 



T. stewardsonii, T. foliata. 



It is to be noted that while brevidens from the position of the 

 expansion falls in this group, in other characters it is more closely 

 related to penita and compacta and thus forms a connecting link 

 between the two groups. It might, perhaps, be considered an ex- 

 ample of development along similar lines arising from a different 

 ancestral stock. It also illustrates the futility of attempting to draw 

 arbitrary lines in any system of classification. Nature does not do 

 things in that way. 



In this arrangement, the synonymy as established by Simpson is 

 followed, with the exception that T. compacta Lea is recognized as 

 distinct from penita Con. Mr. Simpson informs me that, from the 

 examination of additional material since the publication of the 

 Synopsis, he has come to the same conclusion. 



