104 G. LINDSTRÖM, HELIOIJTIU^. 



As represented in the section fig. 4, pl. xi at right in the bacular coenenchyma there 

 is a recurrence of the vesicular one repeated no less than six times more or less distinctly. 

 The thickiiess of the diiferent strata amounts to 1,3 cm. for the central vesicular and to 

 1,1 cm. for the bacular one at the right hand, the small enclosed vesicular bands comprised. 



Coenenchymal gemmation occurs and an instance thereof may be seen on the fig. 

 la, pl. 58 in Milne Edwards' and Haime's British Corals. 



By the gradual development of the diflerent parts we learn, as also has been ascer- 

 tained by other facts, that in the Heliolitida^ the theca is the first part of the coral formed, 

 that it long persists in its original simplicity, that by and by new deposits are added to 

 it and that at last the septa are formed, so that at least in this group of anthozoa there 

 can be no telling that the theca is constructed through the connection of the before 

 existant septa. 



What physiological significance the sudden conversion of the vesicular coenenchyma 

 into the bacular may have, it seems that the coexistense of these two widely different 

 structures in the same specimen may be of phylogenetic value and joint with the knowledge 

 we have gained of the ambiguous nature of Propora? ambigua and its nearest ally Prop.? 

 (Lyellia) discoidea Rom. points to the Proporte as the parents both of the Plasmoporse and 

 the Heliolita;. But Diploepora has been developed in still another direction. It changed 

 its Proporean nature into one, the counterparts of which we detect araongst the Coccoseridfe, 

 especially in the genus Acantholithus. Thence Diploepora ma}' in the systematic arrange- 

 ment fitly be placed last amongst the Plasmoporinse, but as one of them, next before the 

 CoccoseridiB. 



This beautiful and highly interesting species is very common in the Upper Silurian 

 strata, especially in the isle of Gotland. In the lower strata (be) it has only been found at 

 Stora Carlsö. In the higher strata from d to h it occurs at Ryssnäs, Länsa, Dember (all d) 

 in Fårö, along the shores of Fårösund at Wialmsudd, the canal of Stor myr, Rute, Bunge, 

 the quarrj' at Storugns in Lärbro, Vägume vik, Lergraf, Tjelders and Laxarfve in Boge, 

 Hide vik, the shore north of Länna, and the hill of Länua at Slite, at Klints in Othem, 

 Fole, Bara, Follingbo, at Wisby in the strata / — h, and the west coast of Lilla Carlsö 

 stratum /. I have had specimens to examine from Kirna and Ruhde in Estland both 

 belonging to the higher Upper Silurian and also from VVenlock in England. 



I may here remark, that the same conforination of the calicular theca as in Diploepora 

 also seems to prevail in some Proporse, as Pr. tubulata (pl. viii, f. 8, 11, 12) and especiallj^ 

 Pr. speciosa (pl. x, f. 3, 4) where the theca of the mature calicle comes very near to that 

 of Diploepora (pl. ix, f. 34). In Pr. cancellata the younger calicles have completely circular 

 margins (pl. ix, f. 28) while the mature calicle has them of the same type as in Diploepora. 

 As a rule the indented or starshaped theca belongs to a låter stage of growth in a great 

 number of the Heliolitida?. Compare also Pr. conferta pl. ix, f. 11 with pl. viii, f. 33. 



