202 PROCEEDINGS OP THE ACADEMY OP 



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 pi. VIII., fig. 5, b). In these species it is onl}' the ex- 

 treme outer marginals that have their inner cutting point bifid; in 

 H. auriformis a very 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 pi. 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 \Qry like that of Sfeno- 

 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 (pi. VIII., fig. 12) has 27—1—27 teeth with 12 

 laterals, 10 ribs on jaw. 



H. uvulifera (pi. VIIL, fig. ) 23—1—23 with 8 laterals. 



H. auriformis (pi. VIIL, fig. 9) has 20— 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. (pi. VIIL, fig. 4) has 25—1—25 with 11 laterals. 



S. avara. Not examined. 



M. ventrosula (see L. and Frw. Shells N. A. I. 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. Hindsi. Not examined. 



H. Texasiana (pi. .VIIL, fig. 1) has 26—1—26 with 11 laterals. 



H. triodontoides. Not examined. 



H. Mooreana (pi. VIIL, fig. 10) has 20—1—20 with 8 laterals. 



H. tholus. Not examined. 



H. hippocrepis. Not examined. 



H.fastigans (pi. VIIL, fig. 11) has 21—1—21 with 8 laterals. 



ff. Jacksoni. Not examined. 



H. Troostiana (pi. VIIL, fig. 2) has 25—1—25 teeth with 8 



