THE MOLLUSC A 



37 



Molluscs 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 Molluscs 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 ; t', Gastropoda ; D, hamellibranchia ; E, Cephalopoda, a, anus ; a.a, anterior adductor ; 

 c.fif, cerebral ganglion ; /, foot ; /, funnel ; </, 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 



