88 



G. LINDSTRÖM, HELIOLITID^. 



Calicles with exsert edges, the septa, entirely wanting in sorae species, never form 

 coherent laminaj as in Plasinopora and consist of disconnected spines. The tabula3 are 

 generally regular, horizontal, never concave as so comraon with Plasinopora. The coenen- 

 chyma is entirely vesicular with dispersed acul* and baculi, which do not connect so as 

 to form tubuli. There is consequently no aureola, so distinctive for Plasmopora and only 

 in a few instances the septa continne outside the calicles. 



The most prominent characteristics, by which M. Edwards and Haime designated 

 this genus, were that the calicles are exsert, that the septa were inore developed in this 

 genus than in Plasmopora and that they are extended outwards so as to form costse. 

 The first diagnostic is valid for almost all species of this genus, but for the others 

 certainly not. Already in the typical species, Pl. tubulata, calicles occur, even in the 

 same specimen, with external costai together with others quite devoid of such. Several 

 species are without septa, and in those which have such, they are raore rudimentary than 

 in Plasmopora. 



The geological distribution of the species in this genus is as follows. 



Propora tubulata E. H. . . 

 euryacantlia d. . . 

 conferta E. H. . . 

 var. minima n. 

 cancellata n. . . . 

 speciosa Bill. . . 

 compacta n. . . . 

 bacillifera n. . . . 

 ? ambigua n 



Leptsena 

 limeatone. 



Lower 

 Silurian. 



3 g- 



o o 

 t:;' "> 



jia 



Upper Silurian. 



Baeal 

 beds. 



td 



il 



^ 



S"? o' O 



1^ p t^ 



Ludlow. 



Amongst the Heliolitida; this genus contains the oldest known species, which reach 

 deepest down in the strata. Upon the whole, four out of the nine forms of the ProporaB 

 are almost exclusively Lower Silurian or from the lowest U. Silurian and of the other 

 only two ascend to the Ludlow strata of Gotland. Now it is interesting to see how the 

 genus Plasmopora, the younger of these two, nearly related genera, which as far as I am 

 aware is not represented in the Lower Silurian strata, is more developed, has perfect septa 

 and a coenenchyma in which the tubulate structure begins to manifest itself and in some 

 even nearly as completely in Heliolites. With the oider geological age of Propora its more 



