THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE III. VOL. II. 



No. XII.— DECEMBER, 1885. 



I. — On the Synonymy, Structure, and Geological Distribution 



OF SOLENOPOHA COMPACTA, Billings, Sp. 



By H. Alleyne Nicholson, M.D., D.Sc, and Eobeet Etheeidge, jun. 



(PLATE XIII.) 



IN his "Paleeozoic Fossils" (1861-65), Mr. Billings described, 

 without figures, a fossil from the Black River Limestone, to 

 which he gave the name of Stromatopora compacta. The description 

 given was as follows : — " This species forms small subglobular masses, 

 from one to two inches in diameter. The concentric lamella are 

 thin and closely packed together, there being in some specimens 

 from six to twelve layers in the space of two lines" [op. cit. p. 55). 



In 1877, the present writers described, under the name of Tetradium 

 Peachii, a singular fossil which occurred in pebbles of Ordovician 

 Limestone contained in the Old Eed conglomerates of Habbie's Howe, 

 Pentland Hills, near Edinburgh, the source of these pebbles being 

 at that time unknown (Ann. and Mag. Nat. Hist. ser. 4, vol. xx. p. 166). 



In the same year (1877) Dr. Dybowski described a fossil from the 

 Ordovician strata of Esthonia under the name of Solenopora spongi- 

 oides, n.sp., his description and figures leaving no doubt of the 

 identity of this with Tetradium Peachii, Nich. and Etli. jun. ("Die 

 Chaetetiden der ostbaltischen Silur. Formation," p. 124, taf. ii. figs. 

 11a, b). The genus Solenopora was regarded as referable to the 

 Monticuliporoids, and Dr. Dybowski defined it as follows : — 



" Corallum spheroidal ; corallites irregularly prismatical, of very 

 small diameter ; coenenchyma wanting ; tabul<B absent." 



In 1880, we gave a farther and much fuller description of Tetradium 

 Peachii, from specimens which had been collected by Mrs. Robert 

 Gray from the Craighead Limestone (Ordovician) near Girvan, Ayr- 

 shire (Monogi'aph of the Silurian Fossils of the Girvan District in 

 Ayrshire, p. 31, pi. i. fig. 3, and pi. ii. figs. 1, lb). In this descrip- 

 tion we still adhered to the reference of the fossil to Tetradium, from 

 the close resemblance of cross-sections of the tubes in many speci- 

 mens to similar sections of Tetradium, 



Sir J. W. Dawson, in a paper on the microscopic structure of 

 the Stromatoporidce (Quart. Jouru. Geol. Soc. vol. xxxv. ]). 53, 1879), 

 expressed the opinion that the fossil described by Billings under the 



DECADE III. — VOL. II. — NO. XII. 34 



