Distribution. 



Geographical and geological. 



The genera of this family have a world-wide distribution, as well as sonie of 

 the species. In almost all countries where Siliirian and Devonian sti'ata have been 

 seai'ched for them, they have been found. Not to mention Europé and the United States 

 of North America, they have been discovered in the farthest boi^eal parts of North America 

 and Canada, in China, in Sibiria, in Niti in the Himalayan Mountains, in the Caucasian 

 Alps and in Australia. 



The space of time during which they existed, as far as at present known, extends 

 between the uppermost beds of the Lower Silurian (none being found below the Bala 

 beds of England or the Wesenberg strata of Estland), and the Lowest beds of the Upper 

 Devonian formation, after which time all traces of them are löst and they have probably 

 become extinct, and if, as I have stated above, the supposed allied corals in the creta- 

 ceous strata and the recent Heliopora do not link them together, there seems not much 

 hope left to rely on the imperfection of the geological record for explaining their non 

 presence. The Scalpella found in the Upper Silurian of Gotland haviiig no direct relatives 

 before the cretaceous tiraes form a most striking instance of the validity of accepting 

 such an imperfection. But it fails in regard to the Heliolitida;. There is only according 

 to Reuss' descriptions of the Gosau Corals a single species, his Polytremacis Blainvilleana, 

 which in a most deceiving inanner shows tbe characters of the HeliolitidaB. If this in 

 reality is to be regarded as a survival of the Devonian Heliolitidas or not, must at pre- 

 sent be left undecided, as it seems to stånd so isolated and as we must wait for more 

 ample proofs. 



As to the statement of Milne-Edwaeds and Haime (British Foss. Corals, Mountain 

 limestone p. 152) of a Carboniferous Propora, some remarks are to be found in the end 

 of the descriptions of that genus. 



The Heliolitidge are most numerous in the Upper Silurian strata and more so in 

 the northern region of Siluria. In the Lower Silurian beds the Plasmoporinje and the 

 Coccoseridaä prevail, the Heliolitina3 proper thrived chiefly in the Upper Silurian times. 

 The Coccoseridfe which are so nearly related to the Heliolitida^ are in the main confined 

 to the Lower Silurian strata, and only a few remains have been discovered in the lowest 

 Upper Silurians. 



