6 G. LINDSTRÖM, HELIOLITIDiE. 



The State of conservation of the specimens is naturally of the utmost importance. 

 The originally calcareous coralla have in some strata been silicified and consequently 

 their structure is often totally löst or destroyed. But even in those which have retained 

 the calcareous constitution it happens that the older portions have been subject to cherai- 

 cal transformations during their long enclosure in the rocky strata, and the sceletal ele- 

 ments been, as it were, blotted out during that long enduring process. The aspect of 

 the septa and the outlines of the calicles are blurred and evanescent. Such specimens 

 Avould be highly misleading if more perfect specimens were not at hand. But even in 

 well preserved specimens there may sometimes be features and structures, which can be 

 in a certain way described, but, at present at least, l)affle all attempts to declare or ex- 

 plain their real raeaning, as for instance what has been called the formation of fascifp 

 in certain Heliolitaä. 



Generally specimens found in situ are wcll preserved, especially in limestone or 

 marl}^ strata, those, again which are found detached on the shores or on the fields, de- 

 rived or remaniés from older strata are more or less altered, silicitied, changed into do- 

 loraite and weathered. 



In a great number of specimens it is almost impossible by the exteriör appearance 

 to distinguish the species from each other. Apparent similarity of the surface may hide 

 the greatest dissimilarity of the interiör structure. No true knowledge of these corals 

 can be gained without sections in thin, transparent slices, transverse and longitudinal, 

 and chiefly such will give the best results. Tangential ones are also sometimes of value. 

 In some instances it is quite necessary to take a series of many sections along the same 

 line at different depths, as for instance in order to gain a true insight into the phases of 

 the gemmation. 



It is in the deciphering of these microscopic objects of importance not to be de- 

 ceived by false appearances, caused by stränge iniiltered inorganic matter, by diminutive 

 crystals of manganous or ferric oxide hydrate or of arragonite etc. in dispersed individuals 

 or heaped in druses. Below, in the special descriptions, instances will be given how such 

 extraneous matter has been mistaken for sceletal structures and described as such. 



Owing to the great plasticity of these, as well as of so many other corals, it is 

 extreraely difficult to draw neatly the lines of distinktion between the different species, 

 as they seera to be, as it were, confluent with each other and combined by numerous 

 gradations in character. In the descriptions of supposed new species too much stress has 

 been laid upon the size of the calicles, their distance from each other or the amount of 

 the interposed coenenchyma, as being characteristic differences. But who ever has exa- 

 mined a great number of specimens soon has been aware that such characters are of 

 very little value, if any, as large and small calicles may often be found placed at much 

 varying distance from each other on the same colony. In one specimen of Heliol. inter- 

 stinctus, for instance, the calicles vary in size from 1 mm. to 0,5 mm. Of widely greater 

 importance not only for specific, but even for generic differentiation are such sceletal ele- 

 ments as the septa, their shape and size, the structure of the coenenchyma, and the disse- 

 piments as seen in longitudinal and transverse sections. The manner in which the coral 

 has grown may sometimes be of value, especially for singling out local varieties. 



