GENERA OF AMMONITID^. 



85 



liquely round a central axis (fig. 472), so as to form a tur- 

 reted spiral. The shell is also left-handed or " sinistral." 

 In THrrilites, however, the whorls of the shell are in contact, 

 instead of being disconnected as they are in Helicoceras. The 



Fig. 472. — Turrilites catena- 

 tus. The lower figiu-e repre- 

 sents the entire shell ; the up- 

 per figm-e represents the base 

 of the shell seen from below. 

 Gault. 



Fig 473 — a, Ptyclioceios Emeucianiim, reduced — 

 Lowei Gieensand , h, Bacubtet, oiceps, i educed — Chalk; 

 c, Portion of the same, showing the folded edges of the 

 septa ; d, Crioceras cHstatum, reduced — Gault ; e, Scaph- 

 ites cequalis, natural size — Chalk ; /, Hamites rotundus, 

 restored — Gault. 



genns corresponds with TrocJioceras in the series of the Naiiti- 

 liclce. All the species of Ttirrilites are Cretaceous, ranging 

 from the Gault to the Chalk. Cochloceras, of the Trias, 



