530 MM. Nicholson and R. Etheridge, j'un. — 



name of Stromatopora compacta is not referable to the Stromatopo- 

 roids. He states that it " consists of very minute hexagonal tubes, 

 with extremely thin walls and well-developed tabulae, which, from 

 their strong development and continuity, give in some specimens an 

 appearance of concentric lamination." Sir William Dawson regards 

 the fossil as referable to Sienopora {i.e. to the Monticuliporoids). 



Lastly, Mr. A. H. Foord has described (Contrib. Micro. Pal. Sil. 

 Eocks, Canada, Geol. Survey, 1883, p. 24) a variety of this peculiar 

 fossil under the name of Tetradium Peachii, var. Cariadense, Foord, 

 which differs from the British form in having thicker and more 

 wavy tube-walls. Mr. Foord did not observe any tabulas in his 

 specimens, and suggested that the species should be removed from 

 the genus Tetradium. 



So far as we are aware, the above are all the published notices of 

 this singular fossil, the forms described as Tetradium Peachii by us, 

 as T. Peachii, var. Canadense, by Foord, and as Solenopora spongioides 

 by Dybowski being specifically identical with the previously-named 

 Stromatopora compacta of Billings. We have long known that 

 Solenopora spongioides, Dyb., was the same as our Tetradium Peachii, 

 but we are indebted to our friend Mr. Foord for having pointed out 

 to us that the Stromatopora compacta of Billings — which we had not 

 seen — was really the same organism. As it is quite certain that this 

 fossil — whatever its true nature may be — cannot be referred to either 

 Tetradium, Dana, or Stromatopora, Goldf., we shall retain for it the 

 generic name of Solenopora proposed for it by Dr. Dybowski. The 

 species, will, therefore, stand as Solenopora compacta, Bill. sp. Mr. 

 Foord's varietal name of " Canadense " can hardly be retained, as 

 the Canadian specimens are the type of Billings's species. On the 

 other hand, Mr. Foord was quite correct in pointing out that the Scotch 

 specimens differ in some respects from the Canadian type, and we 

 may, therefore, retain for these the varietal name of "Peachii." 

 The Eussian examples, again, appear to agree in all essential respects 

 with the Canadian specimens. To this rectification of nomenclature 

 we may append the following brief remarks on the structure, affinities, 

 and geological distribution of this very remarkable organism. 



Solenopora compacta. Bill, sp., in all its varieties, presents itself in 

 the form of smaller or larger subspherical masses, from the size of 

 a hempseed up to the dimensions of an orange, the external surface 

 being usually lobulate. Even under a good lens, no actual structure 

 can usually be detected as present in the surface itself, but it is 

 common to find indications of a more or less obvious composition 

 out of concentrically - disposed strata. Fractured surfaces have a 

 characteristic porcellanous, sometimes obscurely fibrous aspect, and 

 are usually white or light brown in colour. Thin sections show 

 that the fossil is composed of very minute capillary tubes, which are 

 in close apposition, diverge slightly from a central line, and are com- 

 monly interrupted in their growth in such a way as to give rise to a 

 concentric lamination of the skeleton. The tubes are from a fifteenth 

 to a twelfth of a millimetre in diameter, and are not ''prismatic" 

 or "hexagonal," but, as shown by tangential sections (Pl.XlII.Fig. 3), 



