THE GASTROPODA in 



SUB-ORDER : Rhipidoglossa. This includes all the 

 remaining nacreous gastropods with others that are 

 porcellanous. Palaeontologically they may be divided 

 into two series, according to the (a) presence or (b) 

 absence of the lateral slit (described in Emarginula.} 



(a) Fissttridea resembles Emarginula in shape, but the 

 notch becomes closed in at an early age by the union 

 of the margin and forms a " key-hole " in front of the 

 apex ; as the shell grows this perforation becomes larger, 

 by resorption of the shell, until it occupies the whorl 

 apex and is far from the margin. In Haliotis, the "ormer " 

 of the Channel Islands or "Venus' ear," new notches 

 are formed as the earlier are first closed around and 

 eventually filled in, so that at any time one notch and 

 several perforations are in use. The important fossil genus 

 Pleurotomaria(Fig. 34, a, Sil.-Rec. ) is spirally coiled and has 

 a slit, the filling in of which gives rise to a spiral band, 

 towards which the lines of growth are indented in a 

 V-like manner. The geological history of Pleurotomaria 

 is similar to that of Pholadoniya or Trigonia among 

 lamellibranchs : world-wide in its distribution up to the 

 Cretaceous period, it then became restricted and now 

 survives only in the seas of Japan and the East and 

 West Indies. 



Bellerophon (Fig. 34, b, Sil.-Perm.) is one of the few 

 gastropod shells which is coiled symmetrically, so that it 

 resembles externally a cephalopod shell (but has no 

 internal septa) : the last whorl more or less completely 

 envelops the others, leaving either a very narrow umbilicus 

 on each side, or none. Euomphalus (Fig. 34, c, Sil.-Trias.) 

 is a discoidal shell that is, one in which the spiral angle 

 is increased to 180 or even more, so as to become a re- 

 entrant. 



(b) Among genera without a lateral notch, the most 

 important are Trochus (Sil.-Rec.), a conical shell with 



