238 MOLLUSCA. GASTEROPODA 



nerves come off on each side and extend to the posterior 

 end of the body ; ganglia are poorly or not at all developed. 

 The shell consists of eight plates placed in a longitudinal 

 row on the dorsal surface of the body ; each plate usually 

 overlaps the one behind it, and a flexible band or girdle 

 encircles the whole series of plates. 



All the forms in this group are marine ; they live chiefly 

 in quite shallow water, but a few examples have been 

 found at great depths. Although of great antiquity and 

 represented by a large number of living species, the 

 Isopleura are rarely found fossil. The earliest forms 

 (Priscochiton) occur in the Ordovician ; Helminth ochiton is 

 found in the Silurian ; Gryphochiton in the Carboniferous ; 

 Lepidopleurus, Chiton, and others in the Tertiary. 



SUB-CLASS II. ANISOPLEUHA 



The body is asymmetrical, owing to the twisting of the 

 visceral mass. There are not more than two gills. The 

 shell consists of one piece and is usually spiral. There 

 are two orders, (1) Streptoneura, (2) Euthyneura. 



ORDER I. STREPTONEURA 



In the Streptoneura (or Prosobranchia) the visceral 

 nerve-cord is twisted into a figure of 8. Usually one gill 

 only is present, and it is placed in front of the heart. An 

 operculum is found in most cases. 



Patella. Shell conical, oval or sub-circular ; apex sub-central 

 or excentric, nearer the anterior border, often curved forwards ; sur- 

 face with radiating ribs or stripe, rarely smooth. Margin simple or 

 spinose. Muscular impression horse-shoe shaped, open in front. 

 Jurassic (perhaps Palaeozoic also) to present day. Ex. P. vidgata, 

 Pliocene to present day. 



