246 AN INTRODUCTION TO THE STUDY OF FOSSILS 



in the mantle functions as an anal siphon, for it gives passage to 

 the digestive waste and to the water from the gills, which have 

 been poured into the mantle cavity. It thus corresponds to the 

 slit of the Pleurotomariidae (Fig. 105, st.), to some of the 

 posterior holes in the Pacific coast earshell (Haliotis) and to 

 the posterior canal of such shells as Cypraea. This hole first 

 appears as a notch in the border of the mantle, as in Haliotis, 

 but is surrounded by the later growth of the mantle and, con- 

 sequently, also of the shell. From this aperture are derived both 

 the Latin and common names (La,tm fissiirella, a little hole). 



The fossil forms are now 

 referred to Fissuridea, rang- 

 ing possibly from the Car- 

 boniferous. 



1. Sketch side view, not- 

 ing the anal aperture. 



2. What function did the 

 anal aperture subserve ? 



3. In regard to this aper- 

 ture compare Fissurella with 

 Pleurotomaria, Bellerophon 

 and Haliotis. 



Turrit ella (Fig. 107). 



Trias sic to present. 



Shell shaped like a taper- 

 ing tower or turret, sug- 

 gesting the name (from the 

 diminutive of the Latin, 

 turris, a tower). No um- 

 bilicus persent. There is no 

 siphon and hence the aper- 

 ture of the shell is entire, i.e. 

 not produced into a canal. 



It is represented by many 

 species in the Cretaceous 



A B 



Fig. 107. — Turritclla mortoni Conrad, from 

 the Eocene of Maryland. ( X i) A, shell 

 entire except for the missing apex of the 

 spire. B, the internal mold (or mud fill- 

 ing) of the shell. (After Clark and Mar- 

 tin.) 



