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Prof. J. W. Gregory. 



and the calicular walls are plain, as in the corresponding stages of 

 Heliopora (cf. 7, Plate 13, fig. 4) and some Heliolites [9, Plate 2, figs. 16;, 

 18, and 20]. 



The number of septal ridges and teeth is variable (as in Heliopora) ; 

 the number is from 16 to 20 in F. blainvillei and from 8 to 20 in P. 

 septifera. 



Aureole. — The wall of the calicle m.ay be thin in young and some' 

 internal calicles, but in mature calicles it is greatly thickened. It may- 

 be surrounded by an " aureole " (Lindstrom) of large caeca, with the 

 walls continuous with the septa as in the Heliolitid Plasmopora {cf^ 

 Lindstrom's figure of P. stella, 9, Plate 11, fig. 36). 



Tahulce occur across both the calicles and coenenchymal coeca. 



Baculi. — Rod-like pillars of compact, calcareous material, which 

 Lindstrom has described in the Heliolitidse, occur in P. hlainmlleiy as. 

 remarked by Lindstrom [9, p. 28]. 



4. The Affinities of Polytremacis. 



A. The Relatimis of the Helioporidce and Heliolitid^. 



The preceding account of the structure of Polytremacis shows that 

 the coral consists of a series of tubes, which are marked internally by 

 longitudinal ridges, are crossed by transverse tabulae, and are separated 

 by smaller csecal tubuli. This structure agrees with that of both the 

 living Helioporidse and the Palaeozoic Heliolitidae, and the affinities of 

 Polytremacis are clearly with one or other of those families. 



We have, therefore, to consider the question whether the two 

 families are themselves nearly related. All the older and many 

 recent authorities regard them as intimately allied. Blainville, in 

 1834 [2, p. 392], included them both in the genus Heliopm'a. Dana, 

 in 1848 [4, pp. 539-541], separated them generically, but left them 

 in one sub-family— the Helioporinae. Zittel, in 1879 [20, p. 212], 

 Studer, in 1887 [18, p. 21], and Sardeson, in 1896 [17, p. 353], all 

 included them in one family. Bourne, in 1895 [3], has warmly sup- 

 ported the view of the intimate alliance of the two groups. On the 

 other hand the existence of any special affinity between the Helio- 

 porids and the Heliolitids is denied by Lindstrom [9, pp. 25-26], 

 Hinde [8, p. 87], and Wentzel [19, p. 490]. It is not even always 

 admitted that the corals belong to the same subphylum ; for while 

 Heliopora is unquestionably an Alcyonarian, according to F. Bernard 

 [1, p. 187], the Heliolitidse may be Hydrozoa. 



The proposed separation of the Helioporidae from the Heliolitidse is 

 based on two characters : (1) the presence of true septa in the latter 

 and not in the former ; (2) the absence from the Helioporidse of t\\& 

 calicular theca of the Heliolitidse. 



