HEXACKINIDiE. 



743 



Hg. 22. 



S I X 



Qj <^ 



Fig. 20. Tig. 21. 



b = basals ; fi = radials ; x = special anal plate ; s = syzygy. 



Remarhs. — The Hexacvinidse are closely allied in general structure and 

 habitus to the Platycrinidfe, but are at once distinguished, as already jDointed 

 out in the remarks upon that family, by having a large anal plate between 

 the two posterior radials, and a hexagonal base, the basals thus supporting 

 six plates, instead of five. The base of the Hexacriuidce may be composed 

 either of three plates, or only two ; but in either case the plates are sub- 

 stantially equal, and form a cup with six salient angles, and either two 

 or three re-entering angles at the upper margin. "When the ba.se is bipar- 

 tite, the suture runs from the anal plate to the anterior radial ; but when it 

 is tripartite, the sutures are directed to the anal plate and the right and left 

 antero-lateral radials. In either case, therefore, there is always an interbasal 

 suture running to the middle of the anal side, whereas in the Platycrinidse 

 the nearest interbasal suture to the anal side runs to the middle of the right 

 posterior radial. 



The genera which we refer to the Hexacrinidse were always placed 

 among the Platycrinidce until we separated the two families,* and further 

 investigation only confirms the validity of their separation. It is based on 

 a fundamental structural modification, for which we find an exact parallel 



* Revision, Part III., p. 93. 



