imperfectly-known Species of Stromatoporoids, 17 



the tubercles, either completely concealing these or only allow- 

 ing their ends to be faintly discerned. 



As regards internal structure, the entire coenosteum appears 

 to be composed of approximated parallel horizontal laminae, 

 which are bent into a system of close-set conical elevations, 

 which, in the last-formed layer, give rise to the surface- 

 tubercles. The structures representative of the radial pillars 

 are thus composed of the successively superimposed upward 

 bendings of the horizontal laminas ; and the interspaces between 

 these are occupied by the same laminae bent downwards and 

 closely approximated to one another. 



Ohs. The structure of this form has been well described by 

 Lindstrom and Dybowski. From the description given above 

 and from the accompanying figures (PL II. figs. 7, 8) it will 

 be evident that, supposing the structure to be really what it 

 appears to be, we have to deal with a type exceedingly 

 different from Labechia conferta^ Lonsd,, though the superficial 

 resemblances between the two are very striking. On this 

 point Dybowski is quite clear, and he speaks of this type as 

 the " Oesel'sche Form " of Labechia conferta. The first speci- 

 men which I examined was one kindly sent racby Prof. Ferd. 

 Roemer, and as I found it to be highly crystallized I thought 

 it possible that it might be generically identical with Labechia 

 conferta, Lonsd., and that its apparently very different internal 

 structure might be only the result of extreme mineralization 

 (Mon. Brit. Strom, p. 83). Since then I have collected and 

 examined an extensive series of specimens from the Silurian 

 rocks of Oesel ; and I have come to the conclusion that the 

 present form is unquestionably specifically distinct from L. 

 conferta, Lonsd., and that it is very doubtful indeed if it can 

 be referred to the genus Labechia at all. 



All the specimens which I have seen are in a state of com- 

 plete crystallization internally, though the upper and under 

 surfaces are excellently preserved. This mineralization has 

 not obliterated the internal structure, though it may be 

 assumed to have considerably obscured it. Tangential sec- 

 tions (PI. II. fig. 7) exhibit rows of circular spaces, surroun- 

 ded by a dark line, often exhibiting a dark central spot, and 

 composed of more or less clearly recognizable concentric 

 laminse surrounding this central spot. These circular spaces 

 are about ^ millim. in diameter, and clearly correspond with 

 the cut ends of the radial pillars, as seen in tangential sections 

 of L. conferta, Lonsd. The dark central spot also probably 

 indicates an axial canal. On the other hand, the intervals 

 between these circular spaces are occupied by a dense brown 

 tissue belonging to the coenosteum itself, whereas in L. conferta 

 Ann. ct; Alag. N. HiU. Ser. 5. Vol, xviii. 2 



