28 BRITISH STROMATOPOROIDS. 



ccenosteum is almost always that of a thicker or thinner laminar expansion 

 (PI. Ill, fig. 7), often of large size, epithecate below, and attached by a narrow 

 basal peduncle. In many other forms (such as Stromatopora concentrica, Goldf. , 

 S. typica, Rosen, and Actinostroma clathratum, Nich.) the skeleton is generally of 

 a more massive character, mostly hemispherical or subspherical, the epithecate 

 basal region being reduced in size as compared with the bulk of the organism. 

 Not uncommonly in these more massive species the base is deeply concave, even in 

 very large specimens, and it is sometimes difficult to imagine that the fossil could 

 have been otherwise than quite free. 



All the above-mentioned types of Stromatoporoids are occasionally liable to 

 have their surface of attachment extended laterally, so as to suit the particular 

 foreign body to which they may have been originally attached. Hence specimens 

 may be met with in which the colony has been fixed by the greater part, or even 

 the whole, of the lower surface. All of them also are liable to envelope other 

 organisms which may happen to have attached themselves to their surface or to 

 have grown up alongside of them. Hence it is common to meet with specimens 

 which have grown round, or more or less completely enveloped, colonies of 

 Favosites, Alveolites, Syringoporm, various Rugose Corals, Orthoceratites, Lamelli- 

 branchs, or Gasteropods. It is also common to find that specimens of the 

 Stromatoporoids support upon their surface colonies of Aulopora, Favosites, 

 Alveolites, Thecia, &c. ; these latter, in turn, occasionally supporting a second 

 colony of the same or of some other species of Stromatoporoid. It is, finally, a 

 common thing to find that in some particular locality the Stromatoporoids are 

 particularly liable to grow round and envelope foreign organisms in the way above 

 mentioned, whereas in other localities the same species may be found to manifest 

 very little of this tendency. Thus, in the quarries in the Devonian Limestone of 

 the Schlade-Thal, near Paffrath, the Stromatoporoids seem to have grown round 

 and encrusted almost all the other fossils which occur in the rock ; whereas in the 

 Devonian Limestones in the Bifel the same species (such as Actinostroma clath- 

 ratum, Nich.) are almost always massive and independent. This difference in 

 different localities doubtless depends upon the fact that the local conditions, as to 

 depth of water and the like, were not the same in the two areas, and that these 

 organisms accommodated themselves to the particular environment in which they 

 lived. 



It is also to be borne in mind that the peculiarities above noted with regard to 

 the mode of growth of the Stromatoporoids are by no means special to these 

 organisms. Thus it is quite a common thing for the massive or laminar species of 

 Favosites or Alveolites to attach themselves to foreign bodies, or to surround such 

 extraneous organisms, or to have parasitic colonies growing upon their surface. 

 In neither case do the observed phenomena lend any support to the view that the 



