214 BRITISH STROMATOPOROIDS. 



the centre of one of the astrorhizal systems. These eminences or " mamelons " 

 may be comparatively large, sometimes more than a centimetre in height, in 

 which case they are comparatively few in number. More usually they are smaller, 

 perhaps 2 or 3 millimetres in height, and in this case they are numerous. "When 

 well developed, each of these pointed eminences consists of concentrically laminated 

 tissue traversed centrally by the axial canal of an astrorhizal system, and having 

 the external opening of the same at its apex, while the astrorhizal twigs run 

 down its sides externally. 



The surface presents curious and very puzzling variations in different ex- 

 amples, or in different regions of the same specimen. Sometimes the whole, or 

 a part only, of the surface is covered with minute rounded or elongated tubercles, 

 which sometimes coalesce into vermiculate ridges, and which may have their 

 apices perforated with minute circular apertures. This seems to be the normal 

 condition of the surface. In many specimens, however, this granulated surface is 

 extensively or completely concealed from view by the development of a delicate 

 smooth calcareous pellicle or membrane. This external membrane may pass un- 

 brokenly over the mamelons as well as over the general surface ; but commonly 

 the apices of the mamelons show a few small apertures or the single larger open- 

 ing of an astrorhizal canal. In this latter case the appearances presented remind 

 one of the general surface of Distichopora at points where ampullae are 

 developed. 



As regards internal structure, the general appearances presented by tangential 

 and vertical sections (Plate XXVIII, figs. 2 and 3) are very similar to those of 

 corresponding sections of Stromatoporella eifeliensis, Nich., and need not be more 

 minutely discussed here. The present species is distinguished from S. eifeliensis, 

 as from the other related species of Stromatoporella, by its uniformly inci-usting 

 habit, the development of pointed mamelons, and the characters of its surface. 

 On the other hand, there is a close general resemblance between Stromatoporella 

 curiosa, Barg., and the form which Hall and Whitfield described from the 

 Devonian rocks of Iowa under the name of Coenostroma incrustans (' Twenty- 

 third Ann. Rep. of the State Cabinet,' p. 227, pi. ix, fig. 3, 1873). This 

 latter, as I have shown elsewhere (' Ann. Nat. Hist.,' ser. 6, vol. vii, p. 310, 1891) 

 is really identical with the fossil which I described from the Hamilton rocks of 

 Ontario under the name of Stromatopora nulliporoides (' Second Report on the 

 Palaeontology of Ontario,' p. 78, 1875), this title thus falling to be abandoned. 

 I have now examined an aiithentic example of &vnostroma incrustans, H. and W., 

 and find it to be a species of Stromatoporella, very nearly related to S. curiosa, 

 Barg., in general characters and in minute structure. Upon the whole, however, 

 the American and Canadian type may fairly rank as a distinct species, since it 

 not only shows the superficial distinction that its mamelons are closer set, more 



