THE PEDIGREE OF MOLLUSCS. 159 
The result of these structural dispositions for the history 
of the pedigree of Molluscs, which is confirmed by pale- 
ontology, is that Lamp-shells stand much nearer to the 
primeeval root of the whole tribe of Molluscs than do the 
Otocardia. Probably Mussels and Snails developed as two 
diverging branches out of Molluscs, which were nearly akin 
to the Lamp-shells. 
Mussels, or Plate-gills (Lamellibranchia), possess a bivalved 
shell like the Lamp-shells. In the latter, one of the two 
valves covers the back, the other the belly of the animal ; 
whereas in Mussels the two valves lie symmetrically on the 
right and left side of the body. Most Mussels live in the sea, 
only a few in fresh water. The class is divided into two 
sub-classes, Asiphonia and Siphonida, of which the latter 
were developed at a later period out of the former. Among 
the Asiphonia are Oysters, mother-of-pearl Shells, and fresh 
water Mussels; among the Siphonida, which are character- 
ized by a respiratory tube, are the Venus-shells, Razor-shells, 
and Burrowing Clams. The higher Molluscs seem to have 
developed at a later period out of those without head and 
teeth ; they are distinguished from the latter by the distinct 
formation of the head, and more especially by a peculiar 
kind of tooth apparatus. Their tongue presents a curious 
plate, armed with a great number of teeth. In our common 
Vineyard Snail (Helix pomatia) the number of teeth amount 
to 21,000, and in the large Garden Slug (Limax maximus) 
t.) 26,800. 
We distinguish two sub-classes among the Snails (Cochlides, 
or Gasteropoda), namely, the Stump-headed and the Large- 
headed Snails. The Stump-headed Snails (Perocephala) are 
very closely allied to Mussels (through the Tooth-shells), 
