16 6. LINDSTRÖM, HELIOLlTIDiE. 



all fossil and receiit corals known to me is white or creaincoloured and it looks black 

 only when seen through transmitted light. 



The septa consist of two moieties with fibres tending in different directions and a 

 naiTow whitish partition line. If it is borne in mind that in most palteozoic covals the 

 septa are secreted låter than the theca and by the evidence given by the transverse sections 

 figs. 33 — 34 pl. II and figs. 1, 23 pl. iii it seems that it must be concluded that the septa of 

 the said corals are dependencies of the calicular theca and offshoots of it and not vice versa 

 that the septa form the theca. The smallest septum seen in tig. 33 pl. ii, is without dividing 

 line and has particularly the appearance of having originated from the theca. Sections 

 of the septa nearly resembling those on my plates have been figured by Sardeson (Tabu- 

 laten p. 267) and by Weissbrmel (Zeitschr. deutsch. Geol. Ges. p. 61, 63 fig. 3 & 4.) 

 The latter author has given valuable remarks on the structure of the Heliolitidae, some 

 of which coincide with mine. 



On the interiör edge of the septal laminaj spines of unequal size are often formed 

 growing in the same direction as the elementary fibrillaB and probably owing their origin 

 to lacuna3 left in the tissue of the septal laminte. 



In the Plasmoporinaj the lamina^ are much reduced and consist chiefly of a series of 

 spines and when these, as often happens, have been destroyed the interiör side of the 

 calicle has a fluted appearance through the twelve narrow longitudinal ridges, which serve 

 as the basis of the septa. In some species, as in Propora conferta, the septa have been 

 invested with a multitude of microscopic crystals of intiltred manganous or ferric oxide 

 hydrate giving them a false and deceptive appearance. Such crystals also cover the upper 

 surface of the tabulte in several corals as for instance in Propora conferta (pl. viii hg. 34). 

 It is in some instances ditficult to decide whether one sees organic structures, as acula?, 

 or crystals. 



The longitudinal angular curves of the theca which stånd out between the infoldings 

 of it, give to weathered calicles, which like those of »Lyellia», project free from the coenen- 

 chyma, an aspect like the rugge on the outside of the Rugosa and like the ruga^ they have 

 also been called »costa?» or »pseudocostas» by some authors. Their position in relation to 

 the septa is perfectly the same as in the Rugosa, the septa alternating with these rugte. 

 It must he left as an open question whether this similarity is a sign of relationship be- 

 tween the two groups or not. These rugae of the Heliolitinfe can not be confounded with 

 true costaa, as such really occur amongst the Plasmoporintw 



In the centre of the calicle of Heliolites interstinctus a columella is sometimes 

 formed. It is an isolated narrow, twisted plate. It is, however, not constant, but may 

 dissappear for a while and then again is formed anew. A longitudinal section (pl. i fig. 15, 

 23) shows such changes. 



The size of the calicular areas varies between 4 millimeters and a little less than 

 0,5 millimeter. Their shallowness pi'esupposes a very small amount of animal tissues to 

 fill them and probably the living mäss covering the whole sceleton has been very thin. 



The dissepiment of the calicles consisls in all genuine Heliolitida^ of horizontal 

 tabuktt, regularly distantiated or more or less crowded, horizontal or concave, seldom in- 

 terlaced or crossing each other. At their point of contact with the theca sometimes a 



