THE PEDIGREE OF MOLLUSCS. 1 59 



The result of these structural dispositions for the history 

 of the pedigree of Molluscs, which is confirmed by palae- 

 ontology, is that Lamp-shells stand much nearer to the 

 primaeval 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 

 slieU 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 respiratoiy 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) 

 to 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), 



