DESCKIPTIOIslS OF SPECIES. 73 



axis of the calice. As a result of this twisting, eight septa lie on each side 

 of a plane through the vertical axis of the corallum parallel to the longer 

 transverse axis of the calice. 



In specimens with 24 septa there are three complete cycles in six sys- 

 tems. 



Genus PLATYTROCHUS Milne-Edwards and Haime. 



1S4S. riatytrochiis Milne-Edwards and Haime. Annales sci. nat., .3d ser.,Yol.IX,p.246. 

 1857. Plati/trochiis Milne-Edwards and Haime. Hist. Nat. des Corall., Vol. II, p. 71. 

 1884. Platytrochus P. M. Duncan. Jour. Linn. Soc, Loudon, Vol. XVIII, p. 18. 



The following is the characterization of the genus given in the Histoire 



Naturelle des Corallaires : 



Le polypier est simple, droit, cuneiforme et ue presente aucune trace d'adherence. 

 La columelle est essentielle, fasciculee et termiuee par une surface papilleuse. Les 

 cloisons sont debordantes, larges, tres-peu inc^gales et fortement granulees lat^rale- 

 meut. La muraille est uue, mais presentedes cotes de deux sortes; celles qui occupent 

 le milieu de chaque face du polypier s'elargissent a mesure qu'elles s'elevent; celles 

 qui sont situees sur les c6t6s sont, au contraire, plus fortes et beaucoup plus larges 

 vers la base que pres du calice; de sorte que les bords lateraux du polypier sont 

 presque paralleles. 



The first intimation that this characterization of the genus was faulty was 

 given 1)}^ de Gregorio when he described Platytrochus claibornensis, in his Mon- 

 ographie de la Faune eoc^nique de I'Alabama. In speaking of the columella 

 of that species lie says, "Columella ficta, irregulari, palis eflformata." From 

 a study of longitudinal sections of Platytrochus stokesi and P. claibornensis, I 

 can assert that the columella is not essential, as both Milne-Edwards and 

 Haime and Duncan state. De Gregorio is mistaken when he says "palis 

 eflformata," for there are no true pali. 



On the inner margins of the septa there are small pillar-like lobes. The 

 papillate axis of the corallum, hitherto denominated the papillate termina- 

 tion of the columella, is nothing but these lobes derived from the inner ter- 

 minations of the septa. These lobes are inclined hiward, so that along the 

 inner border of a septum a series of lobes ranged one above another can be 

 distinguished. As any given lobe would increase in height, due to its incli- 

 nation, it would approach an axial position. When the lobes approximate 

 one another closely, they frequently fuse. In some instances appar- 

 ently the ends of opposed septa fuse across the axial space. In Platytrochus 

 there is no essential columella, i. e., the columella is not a separate element 

 of the corallum structure, having an existence independent of the septa. 



