NATURAL SCIENCES OF PHILADELPHIA. 199 
In Mesodon, Triodopsis, and Arionta, the marginals are longer 
than wide, with only two, sometimes bifid cutting points. In 
Stenotrema and Polygyra they are rather wider than long, also 
with two more bluntly bifid cutting points. It must be borne in 
mind, however, that my observations have not led me to believe 
these characters sufficiently constant to be of subgeneric value. 
I prefer to wait till more species have been examined. There is 
also some variation in the mode of passing from lateral to mar- 
ginal teeth, even in the same subgenus. These points will be 
treated more fully under each subgenus. 
Subgenus GONOSTOMA. 
This subgenus is represented in our limits by one species only, 
H. Yatesi, J.G. Cooper, not Pfr., whose jaw and lingual membrane 
are here described. Jaw (pl. XVI., fig. 10) low, wide, slightly 
arcuate, ends scarcely attenuated, blunt; cutting margin without 
median projection; anterior surface with a strong transverse line 
of reinforcement, and numerous, about 12, wide, crowded ribs den- 
ticulating either margin. 
Lingual membrane (pl. IX., fig. 3) long and narrow; teeth 24— 
1—24, with 6 perfect laterals. Centrals with the base of attach- 
ment longer than wide, with expanding lower lateral angles, and 
squarely reflected upper margin; reflection large, stout, bearing 
small but distinct side cusps, with short, blunt cutting points, 
and a long, stout, median cusp reaching the lower edge of the 
base of attachment, beyond which projects the long, acute cutting 
point. lLaterals like the centrals, but unsymmetrical by the sup- 
pression of the inner, lower, lateral angle of the base of attach- 
ment, and the distinct inner side cusp and cutting point. Mar- 
ginals subquadrate (0), a simple modification of the laterals, the 
reflection being more developed, and bearing one inner, oblique, 
long, blunt cutting point, and one smaller side cutting point; the 
extreme marginals (c) are rather wider than high, and the cutting 
points are bluntly rounded. 
The name Yates? being already preoccupied in the genus Heliz, 
Dr. Cooper’s species may be known as Yatesiana. 
Von Martens describes the jaw of Gonostoma as having distinct 
ribs. Moquin-Tandon so figures that of obvoluta, Mull, lenticula, 
Fér, and Rangiana, Fér; and Gassies (Journ. de Conch., XV., 
1867, 15) so describes that of H. constricta, B. 
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