1396 



THE VOYAGE OF H.M.S. CHALLENGER. 



It exhibits all possible passages, from a flatly expanded or nearly discoidal form 

 with a wide open mouth, to a conical or cylindrical, and by constriction of the terminal 

 mouth to an ovate or spindle-shaped form. By complete closure of the mouth the Theo- 

 capsida arise. In some genera remarkable traces of the original triradial structure are 

 preserved. The most interesting of these forms is Axocorys, with an internal axial 

 columella, which bears a number of verticils, each with three radial branches. 



Synopsis of the Genera of Theocyrtida. 



I. Subfamily 



Theocorida. 

 Terminal mouth of 

 the abdomen a 

 simple wide open- 

 ing. 



II. Subfamily 



Theocapsida, 



Terminal mouth 

 closed by a lat- 

 tice-plate. 



No latticed septum between 

 thorax and abdomen. 



An apical horn, 

 No horn on the cephalis, . 

 A complete latticed septum between thorax and abdomen, 



623. Theocapsa. 



624. Tricolocapsa. 



625. Phrenocodon. 



Subfamily 1. THEOCORIDA, Haeckel, 1881, Prodromus, p. 434. 



Definition. T heocyrtida with the basal mouth of the shell open (vel Tricyrtida 

 eradiata aperta). 



