part 4<] THE EVOLUTION OF LIASSIC &ASTKOPODS. 



299 



almost straight in vertical sections of Ceritliium, Avhile in similar 

 sections of Turritella it is markedl}^ oblique. ^ This feature has not 

 been much used in distinguishing the genera. Dr. Cossmann has 

 suggested that it is unreliable ^ ; but we have found the distinction 

 to apply in every gastropod of which w-e have been able to make 

 sections (see fig. 1). 



Fig. 1. — Sections of hirriculate gastropods sliowing tlie characters 

 of the columella. All enlarged ahout 3 times. 



a = Proceritliimn ogerieni (Dumortier), Lower Lias; h — Ceritliium sp., recent; 



c=Tnrritella sp., recent; d=Katosira cf. yottngi (Tate), Lower Lias. 



[La a & h the columella is straight ; in c & d it is oblique. J 



While it is possible to distinguish these several groups in a 

 general way among Liassic species, these latter cannot strictly 

 be referred to the genera Cerithiinn, Chemnitzia, and Turritella, 

 for recent work has shown that these names must be restricted to 

 more modern forms. New generic names, to cover special groups 

 of Jurassic forms, have been proposed b}^ Ralph Tate,^ Gr. Gr. 

 Gemmellaro,'^ and E. Piette,^ and many additional names have 

 been published by more recent workers ; but these names have not 

 been generally used in this country, and it will therefore be 

 advisable to summarize briefly the classification as given by 

 Dr. Cossmann, 6 omitting those details which are not required for 

 the purposes of the present paper. 



Cenacle Cerithiacea Cossmann. 



Family Peoceeithidje Cossmann. 



This family was proposed to include most of the Mesozoic forms 

 Avhich were originally called Ceritliium ; they are generally orna- 

 mented, and have a simple aperture showing an early stage in canal 

 development. 



• 25, pp. 236 et seq. The obliquity of the columella when part of the 

 whorl is filed away is due to the slight helicoid spiral of the columella itself ; 

 thus in Turritella the helix is built up on a helicoid axis. 



2 2, vol. viii (1909) pp. 70-71. 



3 19. ^ 6. 5 25. « 2 & 3. 



