NATURAL SCIENCES OF PHILADELPHIA. 209 



1 in Roemeri: 13 in thyroides: 10 in clausa : 8 in Columbiana:' 

 1 in devia: 10 in profunda: 15 in Sayii: 10 in Mobiliana: over 

 10 in Downieana. 



I have had no opportunity of examining H. divesla, Christyi, 

 Wheatleyi, and jejuna. 



Nothing has been published regarding the jaw and lingual den- 

 tition of the subgenus from species foreign to North America, as 

 it is exclusively confined to this country. 



The jaw of Mesodon does not essentially differ from that of 

 Triodopsis and Polygyra, but may readily be distinguished from 

 that of the other American subgenera. 



The lingual membrane is long and narrow. The general ar- 

 rangement of the teeth is shown in fig. 52. The characters of the 



Fis. 52. 



Lingual dentition of Helix multilineata. 



individual teeth are better shown in my plates. 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 

 (pi. XI., fig. 1), Roemeri (pi. XI., fig, 3), Wetherbyi (pi. XL, fig. 2), 

 Downieana (pi. XI., fig. 4), Saijii (pi. XI., fig. 5) exoleta (pi. XL, 

 fig. ?), Pennsylvanica (pi. XVIIL, fig. 3), Mitehelliana (pi. XVIIL, 

 fig. 5), elevata (pi. XII., fig. 1), Columbiana (pi. XII., fig. 2), 

 Mobiliana (pi. XII., fig. 3), devia (pi. XII., fig. I), profunda (pi. 

 XII., fig. 5) multilineata (pi. XII., fig. 6), dentifera (pi. XII., fig. 

 8), Clarki (pi. XL, fig. 6). Even among these species there are 

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

 cutting points on the subobsolete side cusps of the central and 

 first lateral teeth of Roemeri, Wetherbyi, Downieana, Sayii, ex- 

 oleta, Pennsylvanica, and Mitehelliana. All these species have 

 their side cusp less developed 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 



1 See Ann. N. Y. Lye. of N. H., X. pi. XIV., fig. 2. 



