GASTEKOPODA. 



237 



hundred and one species of venus are at present known to 

 inhabit our seas, and only forty-three species of, this genus 

 have been met with in the tertiary strata ; of cythercea, eighty - 

 five recent species are known, and only fifty-nine tertiary 

 species; of cardium fifty-three recent, and thirty-nine tertiary; 

 ofmytilus, forty- two recent, and fifteen tertiary ; of spondylus, 

 twenty -five recent, and nine tertiary ; of unio, sixty-five 

 recent, and only two tertiary ; from the sub-apennine and 

 the London basin fresh- water formations." 



It appears, therefore, that there has been a constant oscil- 

 lation in the numbers of species in each genus in the tertiary 

 as compared with the modern epoch ; but there has been no 

 gradual perfectioning of the same. 



FIG. 164. Avicula langicostata, Stutchb. 



The GrASTEBOPODA are characterised by the possession of 

 a muscular disc or foot, placed along the under surface of 

 the body, by which they slowly creep. 



