202 PROCEEDINGS OF THE ACADEMY OF 
in Mesodon and Triodopsis, but this character is not constant, the 
marginals of auriculata and auriformis being higher than wide. 
I find considerable difference also between the various species in 
the manner in which the lateral teeth pass into the marginals. In 
auriformis,espiloca and Hazardi, the change is made simply by the 
greater development of the inner cutting point, not by its bifur- 
cation (see pl. VIII., fig. 5, 6). In these species it is only the ex- 
treme outer marginals that have their inner cutting point bifid ; in 
H. aurifornis avery few extreme marginals have a bifid cutting 
point. This species and H. auriculata have very long inner cutting 
points to their marginal teeth (see fig. 9,c of pl. VIII.). In the 
other species examined by me the first marginals have their inner 
cutting point bifid, the transition from laterals to marginals 
being thus very distinctly marked. With these exceptions, the 
dentition of our species of Polygyra is very like that of Steno- 
trema (q. V.). 
The dentition of no foreign species is known with which to com- 
pare our species. 
The count of the teeth in the various species now follows :— 
H. auriculata (pl. VIIL., fig. 12) has 27—1—27 teeth with 12 
laterals, 10 ribs on jaw. 
H. wvulifera (pl. VIII., fig. ) 283 —1—23 with 8 laterals. | 
H. auriformis (pl. VIII., fig. 9) has 26—1—26 teeth, with 8 
laterals. Fig. c¢ shows the proportional greater development of 
the cutting point in the outer laterals. 
H. Postelliana, Bland. Not examined. 
H. espiloca, Rav. (pl. VIIL., fig. 4) has 25—1—25 with 11 laterals. 
H. avara. Not examined. 
H. ventrosula (see L. and. Frw. Shells N. A.J. p. 92, fig. 166), 
93 rows of 24—1—24 with 9 laterals. I have not preserved this 
membrane, so cannot now correctly draw it. 
H. Hindst. Not examined. 
H. Texasiana (pl. VIII., fig. 1) has 26—1—26 with 11 laterals. 
H. triodontoides. Not examined. 
H. Mooreana (pl. VIIL., fig. 10) has 20—1—20 with 8 laterals. 
H. tholus. Not examined. 
H. hippocrepis. Not examined. 
H. fastigans (pl. VIII, fig. 11) has 21—1—2]1 with 8 laterals. 
H. Jacksoni. Not examined. 
H. Troostiana (pl. VIUI,, fig. 2) has 25—1—25 teeth with 8 
