Polytremacis and the Ancestry of Helioporido:. 291 



" Polytremack and tlie Ancestry of Helioporida?." By Professor 

 J. W. Gregory, D.Sc. Communicated by Professor Eay 

 Lankester, r.RS. Eeceived November 21, — Eead Decern-- 

 ber 7, 1899. 



[Plate 2.] 



Contents. 



Sect. PAGE 



1. Introduction .291 



2. Tlie type species of Polytremacis r . . . . 292- 



3. The structure of Poli/tremaeis 293 



4. The afBnities of Polytremacis 294 



A. The relations of the HelioporidfB and Heliolitidae 294 



B. The septal structures 295 



C. The theca of HelioUtes 296 



5. Systematic synopsis and description of new species and a new genus . . . 298 



References 303 



Explanation of plates 304 



1. Introduction. 



The Blue Coral, Heliopora ccerulea (Pall.) is one of the inost isolated 

 of living animals. It is the only known species of its genus, and it 

 has recently been described as the only member of its family. Some 

 Palaeozoic corals have a very similar structure ; but the view that these 

 extinct Heliolitids are allied to the Helioporids is strongly opposed by 

 some eminent palaeontologists. If these authorities be right, then 

 Heliopora is an animal with no close living relations and with no known 

 ancestors. 



The only fossil that has been regarded with any probability as a 

 possible link between Heliopmxt and the extinct Heliolitidae is the Cre- 

 taceous coral Polytremacis. This genus was founded by d'Orbigny in 

 1849, but unfortunately its affinities and structures are still in doubt. 

 " If a genus ever was in need of revision," recently exclaimed Professor 

 Lindstrom [1899, 9, 28], "it is this." Lindstrom indeed suggests 

 that distinct genera are required for each of von Reuss' three species 

 of Polytremacis. 



Without a study of type-specimens in several Continental museums^ 

 a final revision of the genus is impossible for three reasons. The 

 characters of Polytremacis hidhosa d'Orb., the species on which the genus 

 was originally established, are quite unknown; there is one uncertain 

 feature in P. hlaimillei (Mich.), the acting type species ; and doubts 

 have been expressed as to the accuracy of von Eeuss' figures of the 

 specimens which he identified as P. hlainvillei. But in preparing a 

 description of a new species of Heliopora from Somali-land, I have been 

 led to examine the material in the British Museum collection. The 



