698 NOTES ON THE GEODES OF ILLINOIS. 
The form of Helix prevailing in; and which may be said to be 
characteristic of the Quito Valley is Isomeria, peculiar to the 
northern portion of the southern continent. Both Bulimus and 
Bulimulus are essentially characteristic of South America. 
The genus Orthalicus is also South American, but belongs ra- 
ther to the eastern than to the western side of the continent. 
Achatina magnifica, as Pfeiffer suggests, is probably an Orthalicus. 
Oleacina is most numerously represented in Mexico and Central 
America, and the nearest adjacent islands of the West Indies. 
One species is said to inhabit Ecuador, but the exact locality is 
not given. 
-Clausilia is a European genus, but has representatives in Asia, 
and a few species of the sub-genus Nenia in South America,—one 
only in the West Indies, C. tridens Chem. of Porto Rico. Species 
of three operculated genera occur in the Quito Valley,— Cyclo- 
tus, Cyclophorus and Bourciera. The latter, peculiar to Ecuador, 
is placed by Pfeiffer, in the sub-family Realia of the family Cy- 
clostomacea, but it would seem to belong to Helicinacea, with 
which, as pointed ‘out by Troschel, its dentition agrees, although 
in form of shell it is allied to Realia. Cyclotus has several species 
in Mexico and Central America, more in South America, but forty- 
two species in the West Indies, of which thirty-four belong to Ja- 
maica. It may be mentioned that half of the species attributed 
to the valley of Quito, occur also in New Granada. 
Cyclophorus has its principal development in Asia and adja- 
cent islands, but it is curious, as I have elsewhere noticed, that 
while there are a few species found in Mexico, Central and South 
America, seven occur in the West Indies, and all in Guadeloupe, 
Démitnies and Martinique. 
NOTES ON THE GEODES OF ILLINOIS. 
BY PROF. GEORGE H. PERKINS. 
Nor least interesting among the many localities in the Missis- 
sippi valley that attract the geologist and mineralogist is the Illi- 
nois Geode region. This extends for twenty-five or thirty miles 
