PLATE XVII. 



PoTERIOCERAS VENTRICOSUM, F. M'Coy. 



Fig. 1 a. Lateral view of an imperfect specimen, showing the faint longitudinal 

 ridges. 1 b. Longitudinal section showing the septa (dark-tinted spaces) much 

 thickened and obscured by crystalline deposits. 2 a. A fine specimen showing 

 the septa and the greater part of the body-chamber. 2 b. Transverse section of 

 the same, showing the position of the siphuncle. Clane. Dublin Museum of 

 Science and Art. (Page 46.) 



