NATURAL SCIENCES OF PHILADELPHIA. 209 
7 in Roémeri: 13 in thyroides: 10 in clausa: 8 in Columbiana:! 
7 in devia: 10 in profunda: 15 in Saywt: 10 in Mobiliana: over 
10 in Downieana. 
I have had no opportunity of examining H. divesta, Christy 
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 MMesodon 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 
Fig. 52. 
ReRCRER GE a eee u 
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 
(pl. XI., fig. 1), Roémeri (pl. XI., fig, 3), Wetherby? (pl. XI., fig. 2), 
Downieana (pl. XI., fig. 4), Sayz? (pl. XI., fig. 5) exoleta (pl. XI., 
fig. 7), Pennsylvanica (pl. X VILI., fig. 3), Mitchelliana (pl. XVIII., 
fig. 5), elevata (pl. XII., fig. 1), Columbiana (pl. XII., fig. 2), 
Mobiliana (pl. XII., fig. 3), devia (pl. XII., fig. 4), profunda (pl. 
XII., fig. 5) multilineata (pl. XII., fig. 6), dentifera (pl. XIL., fig. 
8), Clarki (pl. XI., 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 Roémert, Wetherbyi, Downieana, Sayti, ex- 
oleta, Pennsylvanica, and Mitchelliana. 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 faras my powers 
1 See Ann. N. Y. Lyc. of N. H., X. pl. XIV., fig. 2. 
