MODE OF PRESERVATION. 29 



Stromatoporoid or the Coral was an habitually " encrusting" organism, such as we 

 see in the recent Hydractinia. 



While the majority of the Stromatoporoids have the under surface largely free 

 and covered by an epitheca, there are, however, other forms which have normally 

 a different mode of growth. Thus certain forms are ordinarily ramose or dendroid, 

 resembling in this respect the ramose species of Pachypora or Alveolites. This is 

 the case, for example, with AmpMpora ramosa, Philh, and Stachyodes verticillata, 

 M'Coy (Plate VIII, fig. 9). 



Lastly, there are species of Stromatoporoids in which the mode of growth is 

 habitually an " encrusting " one. Thus, there occurs in the Eifler-Kalk of Gerol- 

 stein a Stromatoporoid with remarkably large astrorhizee, which usually forms 

 thin crusts, attached by their entire lower surface to the summit of expanded 

 specimens of Heliolites porosa, Goldf., Alveolites suborbicularis, Lam., and Ghcetetes 

 stromatoporoides, Roemer. 1 This species, however, though usually encrusting, is 

 not invariably so, for I have collected examples of considerable thickness in which 

 the under surface has been furnished with an epitheca. Some of the forms which 

 were included by Goldfuss under the name of Stromatopora polymorpha {e.g. 

 S. curiosa, Barg.) appear also to usually form crusts attached to the exterior of 

 corals. This is, further, the case with the form which I described from the 

 Devonian Rocks of North America under the name of Stromatopora nulliporoides 

 (' Second Rep. on the Paleontology of the Province of Ontario,' p. 78). 



Upon the whole, however, it may be unhesitatingly asserted that an " en- 

 crusting " mode of growth, such as we see in the recent Hydractinia, is very unusual 

 among the Stromatoporoids, but that they mostly grow after a fashion very similar 

 to what is seen in the majority of the species of Favosites and Alveolites. 



2. Chemical Composition and Mode of Preservation. 



The Stromatoporoids occur for the most part in limestones, but they are occa- 

 sionally found in argillaceous sediments. They may be regarded, in fact, as having 

 played quite as important a part in the formation of the older Paleozoic Limestones 

 as even the Corals themselves, whole beds of Silurian and Devonian Limestone 

 being often essentially made up of the remains of these organisms. 



The majority of specimens of the Stromatoporoids are composed of carbonate 

 of lime, but it is not unusual, in certain beds, to find specimens in which the 

 skeleton is siliceous. This fact has led some observers to conjecture that the 



1 This curious Stromatoporoid (PI. IV, fig. 2) is very abundant at, the Auberg, near Gerolstein. 

 ]| has been wrongly identified by Eoemer with Stromatopora concentrica, Goldf. (' Lethaca Palaeozoics,' 

 |i 4C0). It is really a species of Slromatoporella, and may be provisionally termed 8. eifeliensis. 



5 



