PALEONTOLOGY 



naiely strong and weak radial ribs, crossed by concentric 

 ridges (growth-lines), except the apex, which is smooth. 

 Internally there is seen a horseshoe-shaped scar, marking 

 the position of the muscles by which the animal pulls 

 the shell tightly down upon the surface to which it 

 clings by its broad, muscular foot. The opening of the 

 horseshoe marks the position of the head. 



The notch at the anterior margin indicates the im- 

 portance of ciliary mechanism in this genus, for it lodges 

 a process of the mantle containing the exhalent aperture. 



FIG. 26. EMARGINULA FISSURA (LINNE), PLIOCENE, (xa.) 



a. Right side view ; a', small portion of surface greatly enlarged to show the 

 ornamentation which is only diagrammatically shown in the other 

 figures ; b, view from above ; f, anterior view. Slit, in b and c, black : 

 at its upper end is seen the callus, and above this the band. (Original.) 



Shell-growth in gastropods takes place as in lamelli- 

 branchs, at the shell-margin and by thickening of the 

 internal layer: the apex of the shell corresponds to the 

 umbo. The presence of a marginal notch, however, 

 causes an interruption to the continuity of the region of 

 growth. Hence to prevent the notch from becoming 

 deeper and deeper as growth proceeds it is rilled up at 

 the inner end by a deposit of shell-substance like that of 

 the inner layer : in time this forms a band from the 

 apex to the slit that in some species of Entarginula is very 

 prominent. 



