NATURAL SCIENCES OF PHILADELPHIA. 



169 



Z. internus (pi. III., fig. 9, b shows the ITth and 18th teeth, c 

 the last tooth). Teeth 28—1—28, with 4 laterals. 



Z. muUidentatus (pi. XYII., fig. 1). The lingual examined had 

 14 — 1 — 14 teeth, with 2 perfect laterals. Morse gives 68 rows 

 with 15 — 1 — 15 teeth, also 2 perfect laterals. 



After my paper was prepared I have had an opportunity of 

 examining the jaw and tongue of Z. Lansingi. It will be seen 

 below that its ribbed jaw and aculeate marginal teeth do not sus- 

 tain my assertion (p. 146) that for the larger divisions these organs 

 may be relied on as systematic characters. The result of my ex- 

 amination of this species was as unexpected as it is puzzling. 



Fiff. 10. 



Jaw (fig. 10, A) low, wide, slightly arcuate; ends scarcely at- 

 tenuated, blunt ; cutting margin without median projection ; an- 

 terior surface with 14 broad, unequal, crowded, flat ribs, slightly 

 denticulating either margin. 



The first impression given by the jaw is that it bears narrow, 

 separated ribs, as in Bulimulus, Cylindrella, etc. A more care- 

 ful study of it, however, shows the ribs to be very broad, crowded, 

 flat, with narrow interstices between them. 



Lingual membrane with 17 — 1 — 17 teeth : 6 laterals. Centrals 

 (fig. 10, B) with the base of attachment longer than wide, the lower 

 lateral angles expanded ; upper margin broadly reflected ; reflec- 

 tion very short, tricuspid ; side cusps decidedly developed, short, 

 bearing distinct cutting points; median cusp long, slender, bulg- 

 ing at sides, reaching nearly to the lower edge of the base of 

 12 



