FOSSIL UNIVALVES 71 
the present, when we have a great number of 
species representing many families (see Figs. 20 
and 21). 
2. Gasteropoda (Greek, gaster=a belly, pous=a 
foot).—Principal features: A univalve shell, gener- 
ally spiral, and a single-lobed mantle. Flat feet. 
Well-developed heads, with eyes, and a tonguelike 
organ called an “‘ odontophore ” (Greek, odontos=a 
Avicula Danby 
(Silurian). 
hie 
Gryphea incurva (Jurassic). Hippopodium ponderosum (J urassic). 
Fia. 20.—Somm Fosstz LAMELLIBRANCHS, OR BIVALVES. 
tooth). Examples: Garden-snails, Whelks, Peri- 
winkles, Limpets, etc. Fossil Gasteropods range 
from Cambrian times. The Cambrian Lingula-flags 
of Wales yield specimens of the genus Bellerophon. 
Nearly 1,300 species have been found in Silurian 
rocks, so you see if you devote yourselves only to 
the study of fossil Gasteropoda you will have your 
work cut out (see Fig. 22)! 
