2 6o PRIMARY FACTORS OF ORGANIC EVOLUTION. 



their motions and secretions. The agreement is so 

 complete as to amount to a demonstration, though in 

 certain cases there may be complications which need 

 additional explanation. 



"A point which may be noted in regard to the Vo- 

 lutidae, to which my attention was called by Mr. Pils- 

 bry, is that in this group the mantle is greatly ex- 

 tended, and there would be more of it to be wrinkled 

 than in such forms as Buccinum, etc. It may be added 



that the forms in which we note the 

 beginning of plaits for this family, 

 many of them, such as Liopeplum 

 and Volutomorpha, had the mantle 

 so extended as to deposit a coat of 

 enamel over the whole shell, as in 

 the modern Cypraea, so that here we 

 have an additional reason why plica- 

 tion should be emphasized in this 

 group. 



"Of course, as before noted, the 

 mechanical principles are the same 

 in any group of gastropods, but 



showing plications of among those j n which the wr i n kling 

 lips. From Dall. 



is confined to the region of the aper- 

 ture, or those shells which are lirate or dentate, as 

 opposed to plicate, several other principles come into 

 play which may be briefly referred to in passing. In 

 the first place, those species which have a very ex- 

 tended mantle, with hardly an exception have a lirate 

 aperture (Oliva, Olivella, Cypraea, Trivia, etc.). With 

 species in which there is a widely expanded mantle 

 and yet no lirations, it will usually be found that the 

 mantle is not entirely withdrawn into the shell in such 

 forms, or is permanently external to the shell (many 



problematica H e i 1 p r i n ; 

 body - whorl opened, 



