THE MOLLUSCA 37 
Molluses shows that the Asiatic and Australian regions are 
separated not by Wallace’s line, but by another line farther east. 
2. Distribution in Time. 
The five classes of Molluses were already differentiated at a 
remote epoch of the Palaeozoic era. The Polyplacophora, the 
Cephalopoda, and Dentalium were represented in the Ordovician ; 
Diagrams of the five classes of Mollusca, from the left side. A, Amphineura; B, Scapho- 
poda; C, Gastropoda ; D, Lamellibranchia; 2, Cephalopoda. «a, anus; @.a, anterior adductor ; 
c.g, cerebral ganglion; f, foot; fu, funnel; g, ctenidium; h, heart in the pericardium ; h.a, 
posterior adductor; m, mouth; pa, pallium or mantle; p.g, pedal ganglion; pl.g, pleural 
ganglion ; ra, radula; st, stomach; st.g, stomato-gastric ganglion ; v.g, visceral ganglion. 
the Rhipidoglossa and the Palaeoconchs (Lamellibranchia allied to 
Solenomya), in the Cambrian. 
On the other hand, many ancient stocks have left no surviving 
descendants. But the appearance of existing genera or families 
supports the phylogenetic conclusions drawn from the study of 
comparative anatomy. Among the Cephalopoda it is the Tetra- 
branchs that appear in the Ordovician; the Ammonites appear 
