MOLLUSCA — GASTROPODS 



245 



1. Sketch view showing aperture and spire; 

 whorls, aperture, slit. 



2. To what is the slit due? 



3. Of what advantage is the slit to the animal ? 



label spire, 



Bellerophon. Ordovician to Penniafi. 



Globular, with the earlier whorls concealed by the later ones. 

 Umbilicus present on each side. Aperture sub-circular with a 

 deep median slit, comparable to that present in the members of 

 the Pleurotomariid^e, which as it progressively closes up with the 

 grow^th of the shell, arches into a keel. 



Especially abundant and widespread in the Mississippian 

 and Pennsylvanian of North America. 



1. Sketch {a) view looking directly into the aperture, {h) side 

 view showing umbilicus. Indicate aperture, umbilicus, keel, 

 slit band. 



2. How is the umbilicus formed ? 



3. A section through the shell shows several whorls. Why 

 are the earlier whorls not visible in the unbroken specimen ? 



4. What characters indicate that this is not a coiled cephal- 

 opod shell ? 



5. What caused the slit band ? its use ? 



Fissurella (Fig. 106). Carboniferous? to present. 



The mantle and shell of this keyhole limpet are pierced by an 



oval hole just over the summit of the visceral cone. This hole 



B 



Fig. 106. — The keyhole limpet, Fissuridea alticosta Conrad, from the Miocene 

 of Maryland. Natural size. A, dorsal view. B, side view. (From Martin.) 



