116 THE AMERICAN MONTHLY [Apr., 



Three ribs prolonged into 3 feet. Ribs and feet latticed. 



Pterocaniuin. 

 Three free wings on thorax. Numerous feet. Pterocodon. 



Free apophyses only on abdomen. 

 Three feet solid, simple. Podocyrtis. 



Three feet solid, ramified. Thyrsocyrtis. 



Three feet latticed. JHctyopodiw///.. 



MOUTH CLOSED. 

 Three lateral wings only on thorax. LitlwrnithiuDi. 



Three lateral wings prolonged into abdomen. Theopera. 



Three wings only ou abdomen. 

 Shell spindle-shaped. Rhopalocanium.. 



Shell 3-sided, j^yramidal. 3 terminal feet. Lithochytris. 



34— Family : PHORMOCYRTIDA. Numerous radial 

 apophyses. 



RADIAL RIBS IN WALL OF SHELL. 



Ribs in thorax and abdomen. Phormocyrtis. 



Ribs in abdomen only. 



Peristome with free terminal feet. Alacorys. 



Peristome smooth, without free feet. Cycladophora. 

 NO RADIAL RIBS IN SHELL WALL. 

 Abdomen cylindrical or ovate, not dilated. C'aJocycliis. 



Abdomen truncate, conical or discoidal, dilated. 



Clathrocylas. 



35.— Family: THEOCYRTIDA. No radial apophyses. 



MOUTH OPEN. 



Abdomen gradually dilated. Mouth wide. 

 Abdomen discoidal. Theocalypira. 



Abdomen conical, one horn. Theoconus. 



Abdomen conical, 2 or more horns. Lopliovoims. 



Abdomen cylindrical. Moutli truncate. 

 One horn. Thorax and abdomen of equal breadth. 



Theocyrtis. 

 One horn. Thorax much broader, abdomen tubular. 



Theosyringitim. 



