MOLLUSCA. GASTEROPODA 249 



(2) the Tectibranchia, which usually possess a mantle, 

 a shell, and a true gill. The following genera are examples 

 of the Tectibranchia. 



Actaeon. Shell oval, ornamented with spiral pitted striae 

 or grooves ; spire prominent, conical, sharp. Aperture elongate, 

 rounded in front ; outer lip sharp ; columella with one strong, 

 slightly oblique fold at the anterior end. Cretaceous to present day. 

 Ex. A. tornatilis, Coralline Crag to present day. 



Avellana. Shell globular, ornamented with spiral striae or 

 grooves ; spire very short. Aperture semi-lunar, curved, entire ; 

 outer lip much thickened, reflected externally, dentate internally. 

 Inner lip thickened, with two or three prominent folds. Cretaceous. 

 Ex. A. incrassata, Upper Greensand. 



Bulla. Shell solid, smooth, sub-globular or ovoid, convolute. 

 Spire concave. Aperture as long as the last whorl, rounded at both 

 ends, widest in front ; outer lip sharp. Inner lip with callus. 

 Cretaceous to present day. Ex. B. ampulla, Recent ; B. globulus, 

 London Clay and Bracklesham Beds. 



The Pteropods are pelagic gasteropods in which the 

 foot is represented by two lateral wing-like fins, and the 

 head is not well marked; a shell may or may not be 

 present. Pteropods occur in large numbers near the 

 surface in the open ocean ; and their shells, which are thin 

 and transparent, form a considerable part of the ' pteropod 

 ooze' — one of the deep-sea deposits found in the warmer 

 parts of the Atlantic Ocean. 



By some writers the Pteropods have been regarded as 

 a distinct Class of the Mollusca ; by others as an Order of 

 the Gasteropoda. Recent work, however, has shown that 

 they agree very closely with the Opisthobranchia, and 

 that they should be regarded as specialised members of 

 the Tectibranch group which have become adapted to a 

 pelagic mode of life. 



