314 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



Its peculiar surface, resembling a scaly coat of mail, when closely examined, is 

 highly characteristic. 



Jaw long, broad, slightly arched, ends blunt, but little attenuated ; with 

 11 broad, stout, crowded ribs, visible on both anterior and posterior surface, 

 and crenulating either margin. 



T. loricata (PI. VII. Fig. J) has over 20 1 20 teeth on its lingual mem- 

 brane ; 8 perfect laterals. 



Genitalia not observed. 



MESODON, EAF. 



Animal as in Patula (q. v.) ; mantle subcentral. 



Shell umbilicated, or with the umbilicus closed, subglobose or orbicularly de- 

 pressed, thin, delicately striate, sometimes decussatedly sculptured ; whorls 5 - 

 6, regular ; aperture rotundly lunar, sometimes narrowed by a small denticle on 

 the parietal wall ; peristome thickened with white, expansively reflexed, its 

 basal margin sometimes unidentate. 



A genus strictly North American, widely distributed over the Eastern Prov- 

 ince, scarcely represented in the Central or Pacific Provinces. It has come 

 down from Post-pleiocene days. 



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

 projection to the cutting margin ; anterior surface with numerous, separated, 

 decided ribs, denticulating either margin. I have counted 13 in M. major ; 10 

 in albolabris; 10 in multilineata ; 11 in Pennsylvania; 12 in Mitchelliana ; 12 in 

 elevata; 13 in Clarki ; 13 in exoleta; 18 in Weiherbyi; 14 in dentifera; 7 in Roe- 

 meri; 13 in thyroides; 10 in clausa ; 8 in Columbiana , -1 7 in devia; 10 in pro- 

 funda; 15 in Sayii; 10 in Mobiliana ; over 10 in Downieana; 10 in Christyi 



and divesta. 



I have had no opportunity of examining M. 

 Wheatleyi, and jejuna. 



Nothing has been published regarding the 

 jaw and lingual dentition of the subgenus from 



species foreign to North America, as it is ex- 

 Jaw of M. Sayit (Morse). 



clusively confined to this country. 



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

 Polyyyra, 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. It will be seen that there are two distinct types of dentition among 

 the species of the subgenus. The first form of dentition is found in albolabris, 

 Roemeri, Wetherbyi, Downieana, Sayii, exoleta, Pennsylvanica, Mitchelliana, ele- 

 vata, Columbiana, Mobiliana, devia, profunda, multilineata, dentifera> Christyi, 



1 See Ann. N. Y. Lye. of N. H., X. PI. XIV. Fig. 2. 



