SURFACE. 59 



epitheoal membrane in the massive species of Favosites, Alveolites, Ueliolites, §c. 

 When such forms are attached to foreign objects, the attachment usually takes 

 place by means of a narrow peduncle ; and if we sometimes find such types to be 

 attached by a wide base, this is only what we also see occasionally in such corals 

 as Favositex gotlandica, Lam., Heliolites interstincta, Wahl., and other similar 

 types. Still, such forms are in no way " encrusting " types ; and we sometimes 

 meet with even largo specimens in which the entire under surface is covered by the 

 epitheca, and is at the same time deeply concave ; so that the primitive condition 

 of attachment to some foreign body appears to have been merely temporary. 



In other forms, such as Act ino stroma clathratum, Nich., an epitheca may be 

 developed ; but more commonly this structure is wanting, and the organism has 

 simply grown in a succession of superimposed strata, applied first to some foreign 

 body and then to one another. In a third group of forms, the organism seems to 

 have been mainly or exclusively " encrusting " in its habit, the entire lower surface 

 being applied to some foreign body, and no epitheca being developed. This is the 

 case, for example, with certain of the so-called Stromatopora polymorpha group of 

 forms (e.g. S.curiosa, Barg.),and is also common, though not universal, in Stroma- 

 toporella cifeh'ensis, Nich. Lastly, in the dendroid types, such as Amphipora ramosa, 

 Phill. sp., and Stachyodes verticillata, M'Coy, sp., the colony resembled that of the 

 ordinary dendroid Corals in being fixed at its base and in having no epitheca. 



(j) The Surface. — The condition of the external surface in the Stromatoporoids 

 can be studied only in specimens in a condition of very good preservation. In some 

 essential respects the surface of any concentric lamina, at any depth, doubtless 

 represents the condition of the exterior ; since each lamina in turn formed for a time 

 the free surface. We are, however, hardly justified in assuming that this is entirely 

 or invariably the case. The most remarkable phenomenon in this connection is 

 the occasional development, in certain specimens, over a part or the whole of the 

 surface, of a thin, apparently structureless, calcareous membrane, largely or wholly 

 imperforate. A somewhat similar phenomenon, though probably one of a totally 

 different significance, is occasionally observed in certain of the Favositidce (e.g. F. 

 tiiberosa, Rom.). Various Stromatoporoids show this curious phenomenon. Thus 

 it occurs commonly in various encrusting types from the Devonian Rocks (Plate II, 

 fig. 14), such as some of those which Goldfuss included under the name of Stroma- 

 topora polymorpha (' Petref. Germ.,' PI. LXIV, fig. 8, d). It is seen in the Stromato- 

 porella (?) nulliporoides, Nich., of the Devonian of North America, and apparently 

 also in the similar or identical " Coenostroma" incrustans, Hall and Whitfield (Plate 

 III, fig. 6). The same thing is seen in Stromatoporella gramdata, Nich., from the 

 Devonian of Canada, well-preserved specimens of which often show over parts of 

 the surface a thin calcareous membrane, pierced at intervals by minute elevated 

 openings (Plate IV, fig. 6). Similar phenomena are observable, not uncommonly, 



