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. 



L Subfamily 



Theocorida. 

 Terminal mouth of 

 the abdomen a 

 simple wide open- 



II. Subfamily 



Theocapsida. 



Terminal mouth 

 closed by a lat- 

 tice-plate. 



Abdomen gra- 

 dually dilated 

 towards the 

 wide open 

 mouth. 



Abdomen 

 cylindrical, of 

 nearly equal 



breadth 

 throughout its 

 whole length. 

 Mouth trun- 

 cate. 



horn on the \ 



cephalis. I Thorax much broader than 



[ the tubular abdomen, . 



Two horns or a bunch of horns on the 

 cephalis, ...... 



No horn on the cephalis, .... 



Abdomen 

 ovate or in- 

 versely conical. 

 Mouth more or 

 less con- 

 stricted. 



A single apical 

 horn on the - 

 cephalis. 



Shell-cavity without an in- 

 ternal columella, . 



Shell-cavity witli an axial 

 columella, 



Two horns or a bunch of horns on the 

 cephalis, ...... 



No horn on the cephalis. 



An apical horn, 

 No horn on the cephalis, 

 A complete latticed septum between thorax and abdomen, 



No latticed septum between 

 thorax and abdomen. 



616. Theosyringium. 



617. Lopliocyrtis. 



618. Tricolucampe. 



619. Theocorys. 



620. Axocorys. 



621. Lopliocorys. 



622. Tlieocampe. 



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). 



