172 BRITISH STROMATOPOROIDS. 



From 8. concentric a, Goldf., the present species is distinguished by its much 

 finer skeleton-fibre and the greater delicacy of the ccenosteal tissue resulting 

 from this. The ccenosteum is, further, distinguished by its hemispherical or dis- 

 coidal form, and the presence of a basal epitheca ; while the zooidal tubes are 

 closer, more regular, and more abundantly furnished with tabulae than is the case 

 with the former. From both S. Hupschii, Barg., and S. discoidea, Lonsd., the 

 present species is distinguished, among other characters, by the comparative fine- 

 ness of the skeleton-fibre and the greater delicacy of the reticulated skeleton. In 

 8. discoidea, moreover, the astrorhizal system is extraordinarily developed. From 

 8. Carteri, Nich., lastly, the present species is separated by its more delicate 

 skeleton-fibre and the much less lax and open character of the skeletal network ; 

 while the former is destitute of astrorhizae, or has these structures developed in 

 the feeblest manner. 



Stromatopora typica, Rosen, though a very abundant and very widely distributed 

 form, seems to have been commonly overlooked by palaeontologists, and I have 

 therefore little to say as to its synonyms. I have examined von Rosen's original 

 specimens in Dorpat, and I have collected many similar ones in the Silurian Rocks 

 of Esthonia ; so that I have no doubt as to the precise species this observer had in 

 view, even if his excellent figures had not placed this beyond doubt. My friend 

 Mr. J. F. Whiteaves has been good enough to send me a fragment of the original 

 specimen of the " Gaunopora " Rudsonica, described by Sir J. "W. Dawson (' Quart. 

 Journ. Geol. Soc.,' vol. xxxv, p. 52, pi. iv, fig. 9, and pi. v, fig. 10, 1879) from 

 the Silurian Rocks of Hudson's Bay, together with another and much better pre- 

 served fragment of the same species from the Silurian of Cape Churchill. The 

 microscopic examination of these fragments has shown that this form is a true 

 Stromatopora, with very close relationships to 8. typica, Rosen. The general 

 character of the skeletal network is precisely similar to that of S. typica, except, 

 perhaps, that it is a shade coarser than is usual in the latter species ; while the 

 minute structure of the skeleton-fibre is identical in the two. There are, in fact, 

 only two apparent points of distinction between 8. Hudsonica, Daws, sp., and 

 8. typica, Rosen, to which any importance could be attached. One of these is 

 that in the former the astrorhizae are always regularly superimposed in vertical 

 rows, each system being connected with a wall-less axial canal of comparatively 

 large size. Bach astrorhiza, therefore, opens on the surface of the latilamina to 

 which it belongs by a comparatively large circular aperture, corresponding with 

 the axial canal, this aperture being placed at the summit of a minute pointed 

 eminence. The surface thus shows numerous small, regularly placed "mamelons," 

 corresponding each with the centre of an astrorhizal system. In this character, 

 as pointed out by Dawson, S. Hudsonica resembles the form described by Hall 

 and Whitfield (' Twenty-third Ann. Rep. on the State Cabinet,' pi. ix, fig. 3, 



