vii MOLLUSCATHE ASYMMETRY OF THE GASTROPODA 159 



not become coiled forward, the original posterior position of the pallial complex 

 being retained as the most favourable under such circumstances. As an example 

 of this we have the Nautilus, all Nautiloidea and Ammonitidea, with their exogas- 

 trically (anteriorly) coiled shells and posteriorly placed pallial complexes (Fig. 137). 

 The coiling of the shell of Spirula forms an exception to that of all other Mol- 

 lusca, being endogastric. With regard to this we have to consider first, that the 

 shell of Spirula is internal and rudimentary, and that the backward coiling does not 

 in any way affect the posteriorly placed mantle cavity ; and second, that only the 

 modern genus Spirula has such a shell. The Miocene genus Spirulirostra has its 

 phragmacone endogastrically bent but not coiled, and the older Belemnitidce never 

 have either curved or coiled shells. Moreover, the shell of this whole group, being 

 internal and, as far as the original purpose of a shell, protection of the body, is con- 

 cerned, rudimentary, does not come under consideration in the present discussion. 



15. 



In an animal living in mud, like a limicolous bivalve, there appears no reason 



0*0 



FIG. 139. Hypothetical transition 

 form between Dentalium (Pig. 138) 

 and the racial form of the Gastropoda 

 (Fig. 140), from the left side. 



FIG. 138. Dentalium, diagram from 

 the left side, g, Genital gland ; kt, 

 cephalic tentacles. 



FIG. 140. Hypothetical racial form 

 of the Gastropoda, from the left side. 



why the shell should not simply become elongated, and why the mantle cavity and 

 pallial complex should not retain the posterior position. Dentalium (Fig. 138) is 



