60 FOSSIL MOLLUSCA OF THE CHALK. 



None of the group of the rotundati have been found with the produced mouth like the 

 Turrilites, and the only modification known in the adult shells of that group is a thicken- 

 ing of the edge of the mouth : probably this will be found to be another generic difference 

 between Turrilites and Helicoceras. 



Classification of Turrilites. 



1. Rotundati, D'Orb. Whorls round; siphuncle near the middle of the exterior side 



of the whorl ; ribs simple ; mouth thickened. 

 Common in the Gault. 

 T. acuticostatus, D'Orb. C. C. 



2. Angulati, D'Orb. Whorls angular; siphuncle near the suture; mouth covered 



with a hood. 



2 a. Lateral lobe trifid ; ribs or tubercles not arranged in regular lines 

 across the whorl. 

 T tuberculatus, Bosc. L. Chalk. 

 T. Gravesianus, D'Orb. PL 144, fig. 3. L. Chalk. 

 T Mantelli, Sharpe. 



2 a a. Lateral lobe trifid; tubercles in regular rows across the 

 whorl. 

 T Bergeri, Brong. Gault and L. Chalk. 



2 b. Lateral lobe bifid ; tubercles or ribs arranged in regular lines 

 across the whorl. 

 T. catenatus, D'Orb. PI. 140, fig. 1. Gault. 

 T. Puzosianus, D'Orb. PI. 143, fig. 1. Gault. 

 T. Scheuchzerianus, Bosc. L. Chalk. 



T costatus, Lam. L. Chalk. 

 T Wiestii, Sharpe. L. Chalk. 



2c. Lateral lobe bifid; tubercles not in regular rows across the 

 whorl. 

 T Morrisii, Sharpe. U. G. Sand and L. Chalk. 



