134 G. LINDSTRÖM, HELIOLITID^T!. 



As to the growth of tlio theca in tho genus Propora I may liere remark tliat in several spccies cliangcs 

 occur in a manner soraewliat similar to vvliat happens in l)i]iloepnra. Tlie initial sharpedged circular margins 

 assume tbe shape of a ring of small beads, wliicli form the basos of trabeculoe (baculi), out of whicb, twelve in 

 number, by and by, tbe tbeea is niado up in tbe låter stage of grnwtb. Tbese trabeculff of tbe eommon tibrillous 

 strueture lic elose against eaeb otber and as niay be seen from weatbered specimens wbcre the calieular tubes 

 stånd free, they form a compact tbeca, although it is very tbin between tbe baculi. Soraetimes tbere is seen 

 as long, narrow slits in tbe theca, but this may be due to disintegration tbruugh pression or låter changes. In 

 plate XI, fig. 2 tbere is perhaps a side view of a wall with its trabecubT. It is remarkable that in tbe oldest 

 ProporsE as P. eonferta nnd bacillifcra this strueture of tbe tbeca is not so prominent as in tbe Upper 

 Silurian oues. 



