KONGL. ÖV. VET. AKADEMIENS HANDLINGAR. BAND 32. N:0 I. 15 



sceletou of tlie polyp. It is quite certain that in the majority of the palaaozoic corals and 

 chiefly amongst the Rugosa ^ the theca is secreted before any septa are formed. The 

 same is the case in composite colonies of Heliolitidte, for instance in Diploépora, where 

 young calicles just beginning to sprout forth from the coenenchyma on the top of the 

 branches show a nan'ow, smooth edge forming the circular Avall without the least traces 

 of any septa. By and by there are formed feint infoldings in the margin, becoming 

 more and more distinct and assuming the shape of well developed septa with spiny edges. 

 Pl. X iig. 32—35. 



There are true septa, in no way homologous with the pseudosepta of Heliopora. 

 They are without exception always and constantly twelve. They may be more or less per- 

 fectly developed or even totall}' wanting in some varieties as Heliol. decipiens, though 

 their presence in some calicles of the same specimen indicates that they may have been 

 present in all, but have been absorbed. In the pl. ii figs 3 — 6 an illustration is given of 

 this condition from several closely set calicles on the same surface of a Heliolites. The 

 calicle, fig. 3, with complete septa, ineeting in the centre, is evidently one of the oldest, 

 being smaller, next, fig. 4, one with somewhat lesser septa, then, fig. 5, one with much 

 reduced septa like those common in Heliol. interstinctus and at last, fig. 6, a calicle, like 

 the plurality of the other calicles on the same surface, without any septa at all and 

 giving the characters of Heliol. decipiens. 



In some species of the genus Heliolites there are septa of alternating size (pl. ii 

 f. 29, Hel. porosus), as if indicating a tendency to form two cycles of six septa each. 

 But we have as yet no evidence that the six longer septa have originated earlier or prior 

 to the shorter ones, nor does the development, at least as seen in the coenenchymal gem- 

 mation, confirm this supposition. 



The shape of the septa varies much as to the different tribes. Common to thein 

 all is that they are thin laminse, intimately composed of minute, microscopic fibrilte di- 

 rected upwards and inwards. Pl. ii fig. 35, pl. iii fig. 2. They have consequently quite 

 the same microscopical structure as the septa of almost all other corals and even con- 

 cordant with the structure of all other vertical elements both of the Heliolitas and others. 

 In a longitudinal section of Heliol. porosus (pl. ii fig. 35) this is very distinctly seen, where 

 a calicular theca meets a theca of a coenench3^maI tube and between them the dark line, 

 which Miss Ogilvie calls centre of calcification, but the true nature of which is not ex- 

 plained by her suggestions. Next we have two figures (pl. ii fig. 34, pl. iii fig. 1) of 

 transverse sections representing parts of calicles with septa and coenenchymal tubes. Each 

 tube has a theca well circumscribed through a dark line, dividing it from the sur- 

 rounding thecas. The theca and the septa of the calicle are coherent as a whole and se- 

 parated from the coenenchyma through the dark line. The true colour of this line, in 



^ There is no valid reason, as it seems to me, at least in the present provisional state of our know- 

 ledge of the fossil corals, to abandon this name for those corals, which låter authors have called Tetracorallia or 

 Pterocorallia. The former name is not borna up by the structure, as the arrangement of the septa according to 

 four primary ones is visible only in a few species. The denomination Rngosa again is most characteristic 

 pointing to one of the chief distinctions of this group. There exist in none of them costse, but the exteriör 

 longitudinal foldings of the wall coupled with the peculiar initial stages of growth separates them from all 

 posterior corals. 



