THE GASTROPODA 117 



Fusus has been described; allied to it is Clavella 

 (Fig. 35, b), an abundant Eocene form, now represented 

 by a single Polynesian species. It differs from Fusns in 

 being smooth and in the shape of the whorls, which have 

 rather flat sides terminated above by a horizontal shelf ; 

 Sycum, or Leiostoma (Eocene), is another smooth form, 

 differing in its much shorter spire and less clearly 

 denned canal. 



The Volutidce (Cret.-Rec.) consists of somewhat fusi- 

 form shells, though without the sharp demarcation of the 

 anterior canal seen in Fusus, the aperture being long and 

 somewhat parallel-sided. Their special feature is the 

 development of spiral plaits or ridges on the columella, 

 which define grooves along which slide the tendons by 

 which the animal pulls itself into the shell. The com- 

 monest fossil genus is Volutilithes (Cret.-Rec.), a richly 

 ornamented form, abundant in the British Eocene 

 (Fig. 35, c), but now represented by a single South African 

 species. Aurinia, or Scaphella (Fig. 35,^), a smooth form 

 with very rounded apex, is found in the British Pliocene, 

 and now lives in warmer seas. 



Another fusiform shell with aperture shaped as in the 

 Volutidce is Pleurotoma (Cret.-Rec.), in which there is a 

 posterior canal which takes the form of a notch at a little 

 distance from the suture (Fig. 35, d), recalling the lateral 

 slit of Pleurotomaria, and like that causing an inflection of 

 the growth-lines, but not closed by a special secreted band. 

 This abounds in species in the British and other Tertiaries, 

 and in the warm seas to-day. Conorbis (Eocene) is similar, 

 but with narrower aperture, forming a transition to Conus 

 (Cret.-Rec.), with very short, depressed spire, narrow, 

 parallel-sided aperture, and posterior canal close to the 

 suture : its distributional history is like that of Pleurotoma, 

 but it is more closely restricted to tropical seas to-day. 



Brief mention must be made here of a specialized 



