298 



Prof. J. W. Gregory. 



has broken down. The only escape from this difficulty is the heroic 

 coin-se of dividing Polijtremacis into two unrelated divisions, one species 

 being regarded as an isolated, belated survivor of the Heliolitidse, and 

 another as a premature ancestor of the Helioporidse. 



Polytreiaacis agrees with the Heliolitidse by many remarkable points 

 •of structure, such as the presence of the aureole, the closure of dead 

 calicles by coenenchymal overgrowth, and the inconstancy of the septa 

 in the lower parts of the calicles. Polytremacis is allied to Heliopom by 

 equally striking points of resemblance, such as the fluted calicular 

 walls, with their numerous, irregular, septal ridges, the granular 

 external surface with its circumcalicular ring of septal teeth. On the 

 axiom that things that are allied to the same are allied to one another, 

 the close affinity of Heliopora and Heliolites seems more probable than 

 some palaeontologists are inclined to admit. Heliopom, in fact, may 

 have descended from the Heliolitidse by the reduction in size and conse- 

 quent increase in number and in variability of arrangement of the 

 •coenenchymal caeca. 



5. Systematic Synopsis. 



ALCYONAEIA. 



Order. — CdNOTHECALiA, Bourne. 



Family 1. — Heliolitid.e. 



Definition. — Coenothecalia with regular, well-developed septa, gener- 

 ally 12 in number in each calicle. 



For subdivisions, see Lindstrom [9, pp. 35-37]. 



Family 2. — Helioporid.e. 



Definition. — Coenothecalia, with small, irregularly arranged coenen- 

 chymal caeca, and a variable number of septa or septal ridges. 



Genus 1. — Heliopoea, de Blainville, 1834. 



Definition. — Corallum of thick lobes or digitate fronds. Calicles thin 

 walled. Septal ridges numerous, and always short. 



Type Species. Millepora ccermlea, Pallas, 1766. 'Elench. Zooph.,' 

 p. 256. 



Heliopora ccmdea, Blainville, 1834. 'Man. Act.,' p. 392. 

 Eecent. Indian Ocean. 



Species 1. — Heliopora somaliensis, n.sp. 



Characters. — Corallum massive. Calicles very small, being between 

 0-5 and 1 mm. in diameter. They are about 2 mm. apart. From 

 12-15 septal ridges, which are sometimes prominent and well- 

 developed. Coenenchymal caeca circular. (Plate 2, figs, M-c.) 



