EASTERN PROVINCE INTERIOR REGION SPECIES. 295 



narrowed by a small deuticle on the parietal wall; peristome thick- 

 ened with white, expansively reflexed, its basal margin sometimes 

 unidentate. 



A genus strictly Korth American, widely distributed over the East- 

 ern Province, scarcely represented in the Central or Pacific Provinces. 

 It has come down from Post-Pliocene days. 



Jaw stout, high, arcuate, wide, ends but little attenuated, blunt; no 

 median projection to the cutting margin ; an- ^"- •^''• 



terior surface with numerous, separated, de- 

 cided ribs, denticulaling either margin. I 

 have counted 13 in M. major ; 10 in alholabris ; 

 10 in multilineattis ; 11 in Pennsylvanicus ; ] 2 in Ja^^ of ^- -saj/"- (Morse.) 

 Miichellianus ; 12 in elevatus ; 13 in Clar'ki; 13 in exoletns ; 18 in Weth- 

 crhyi ; 14 in dentifenis; 7 in Eocmeri; 13 in thyroides; 10 in claustis; 

 8 in Golumhianns ; 7 in devitis ; 10 in profund^is ; 15 in ^Sayiij 10 in 

 Mohilianus ; over 10 in Downieanus ; 10 in Christy i and divestiis. 



I have had no opportunity of examining M. Wheatleyi and jejumis. 



Nothing has been ijublished regarding the jaw and lingual dentition 

 of the subgenus from species foreign to o^orth America, as it is exclu- 

 sively confined to this country. 



The jaw of Mesodon does not essentially differ from that of Triodopsis 

 and PoJygyra, but may readily be distinguished from that of the other 

 American subgenera. 



The lingual membrane is long and narrow. The general arrangement 

 of the teeth is as in Patula. The characters of the individual teeth 

 are shown on my Plate VIII of Terr. Moll., V. It will be seen that 

 there are two distinct types of dentition among the si)ecies of the 

 subgenus. The first form of dentition is found in albolahris, Boe'meri, 

 Wetherhyi, Doicnieanus, Sayii, exoletns, Pennsylvanicus, Miichellianus, 

 elevatus, Colnmhianus, Mohilianns, devius, profundus, mulUlineatns, denti- 

 ferus, Christyi, divesius, ClarJci. Even among these species there are 

 some important variations. Thus, I have failed to detect any side cut- 

 ting points on the subobsolete side cusps of the central and first lateral 

 teeth of Eoemeri, Wetherhyi, Ihivnieanus, Sayii, cxoletus, Pennsylvanicus, 

 and Mitchcllianus. All these si)ecies have their side cusp less devel- 

 oped than in the other species mentioned above. The presence of 

 the cutting point may be detected by better manipulation than I am 

 able to give, but as far as my powers go I cannot find it. The large 

 median cutting point, however, has a decided lateral bulging, which is 



