122 THE VOYAGE OF H.M.S. CHALLENGER. 
seems to differ from all other Alcyonarians except Corallium. From both Corallium and 
Tubipora, Heliopora differs in that the hard tissue of its corallum shows no signs of 
being composed of fused spicules, but in its histological structure most closely resembles 
Zoantharian corals. With the Milleporide and with Pocillopora and Seriatopora 
Heliopora is allied solely on account of its possession of tabula, and these structures 
being possessed alike by Hydrocorallinee, Helioporidee, Tubiporide, and certain Madre- 
poraria, their presence is proved to be of no classificatory importance, and is of less 
value even than Professor Verrill showed it to be. The group Tabulata must be 
entirely given up as only misleading in its signification, and the corals formerly placed 
in it must be distributed amongst their natural allies. There can hardly be a doubt that 
Seriatopora will prove to be, like Pocillopora, a Zoantharian. Heliopora thus stands 
quite alone amongst modern forms; and in the peculiar structure of its cellular 
ceenenchym it is so remarkable that it is unlikely that on examination of the soft parts 
of other living corals, at present known from their coralla only, any near relatives of it 
will be discovered. Amongst extinct forms, however, Heliopora has several close allies, 
and the genus itself existed in the Cretaceous period. The genus Polytremacis differs 
apparently only in the more perfect development of the so-called septa, which reach to 
the centres of the tabule. The genus occurs in the Chalk, Greensand, and in Eocene 
formations. Heliopora has, further, a very closely allied paleozoic representative in 
Heliolites, in which the ccenenchymal tubes are provided with very closely placed 
tabula. Professor Alleyne Nicholson' groups with these Plasimopora, Propora, Lyellia, 
and Pinacopora ; he finds a ditticulty in the fact that the cavities of the tubes do 
not communicate with those of the calicles in Heliolites, and appears not to have 
understood my description of the manner in which the polyp cavities communicate 
with the sacs of the tube cavities in Heliopora. There are no apertures in the walls 
of the calicles, or tubes in Heliopora, any more than in Heliolites, and the connecting 
canals pass, as described, only over the edges of the mouths of the coenenchymal tubes, 
lying quite superficially. 
The three genera Heliopora, Polytremacis, and Heliolites differ from one another in 
so slight a degree that they are placed under the one genus Helopora by Queenstedt. 
For the reception of the genus Heliopora and its fossil allies I formed a separate family 
of Alcyonarians characterised as follows from the recent species. 
1H. Alleyne Nicholson, On the Structure and Affinities of the Tabulate Corals of the Palwozoic Period, Edinburgh, 
1879, pp. 242, 243. 
