12 BRITISH STROMATOPOROIDS. 



and the mere presence of such canal-systems does not, therefore, afford a sufficient 

 ground for generic distinction. Indeed, it occasionally happens that certain 

 individuals of a given species exhibit such " astrorhizas," while in other individuals 

 of the same species these structures are wanting, or are, at any rate, not con- 

 spicuous. I am, therefore, of opinion that the genus Ccenostroma, Winch., cannot 

 be retained with advantage. 



One of the most important contributions to the study of the Stromatoporoids 

 is that published by Baron von Rosen in 1867, under the title ' Ueber die Natur 

 der Stromatoporen, und iiber die Erhaltung der Hornfaser der Spongien im fossilen 

 Zustande.' In this work, the author recounts the results of an investigation into 

 the structure of the Stromatoporoids by means of thin sections prepared for the 

 microscope ; and the value of his memoir is further enhanced by a number of 

 excellent plates, dealing principally with the minute structure of the skeleton. 

 The material upon which von Rosen based his work was derived from the Upper- 

 Silurian Rocks of the north of Europe, from which he describes several new 

 species. 



Having recently had the opportunity of examining in Dorpat the original 

 specimens and slides upon which von Rosen founded his species, and having 

 myself collected a large series of the same forms, I shall be able later to discuss 

 more fully the characters and affinities of these species. In the meanwhile the 

 following brief remarks may be made as regards some of them. 



Stromatopora typica, Rosen (op. cit., Taf. I, fig. 1), is a species common in the 

 Wenlock Limestone of Britain, and is a true Stromatopora (in the sense previously 

 defined). The type-specimen of Stromatopora astroites, Rosen, has its internal 

 structure almost destroyed, as the result of crystallisation ; but other specimens 

 included by Rosen under this name are apparently identical with S. typica, and 

 the specific name of astroites must therefore bo abandoned in favour of typica. 

 Stromatopora elegans, Rosen, though much crystallised, appears to be identical 

 with S. discoidea, Lonsd., the latter name having the priority. Stromatopora 

 Schmidtii, Rosen, is a very peculiar type of the genus Actinostroma. The 

 Stromatopora variolaris of Rosen is a species of Glathrodictyon, and is of common 

 occurrence in the Wenlock Limestone of Britain. Stromatopora regularis, Rosen, 

 is also a species of Clathrodictyon ; and is also found, though rarely, in the 

 Wenlock Limestone of Britain and of Gotland. The remarkable type described 

 under the name of Stromatopora dentata appears to be properly referable to a new 

 genus allied to Labechia, E. and H., which I shall name Bosenella. To this genus 

 also belongs the species described as Stromatopora Ungeri. It may be added that 

 von Rosen devotes a section to the discussion of the characters of S. pohjmorpha, 

 Goldf., and, rightly, concludes that Goldfuss had included several types under this 

 specific name. 



