GENUS DICTYOSTROMA. 85 



Labechia in the very rudimentary condition of the radial pillars, and also in the 

 correspondingly increased development of the vesicular laminse. In Labechia (Fig. 

 5), the coenosteum consists of strong, "continuous" radial pillars, separated and 

 united by curved vesicular plates which carry no tubercles. In Rosenella, on the 

 other hand, the coenosteum (Plate VII, fig. 12) consists wholly of curved vesicular 

 plates, which are not traversed by continuous radial pillars, but have the whole of 

 their upper surfaces covered with rudimentary pillars in the form of conical 

 tubercles. Tangential sections (Plate VII, fig. 13) show sometimes the cut edges 

 of the curved vesicular plates, sometimes the transversely divided tubercles which 

 spring from these plates, and sometimes the porous tissue of the plates themselves. 

 The type-species of the genus is remarkable for the large size of the elongated cells 

 which form the coenosteum, single vesicles being sometimes an inch or more in 

 length ; but in other species (e.g. R. dentata, Rosen) the vesicles are much smaller. 

 There are certain forms of the genus which show a singular transition between this 

 and Clathrodictyon ; but I shall be able to speak more definitely on this point 

 later on. 



Genus Dictyostroma, Nich. 

 (' Palaeontology of Ohio,' vol. ii, p. 254, pi. xxiv, fig. 6, 1875.) 



This genus was founded by me for the reception of a singular Stromatoporoid 

 (Dictyostroma undulatum) from the Niagara Limestone of North America. As I 

 unfortunately prepared at the time no thin sections of this form, and now possess 

 no examples of it, the genus cannot be regarded as being adequately defined or 

 satisfactorily established. The merely macroscopic characters of D. undulatum, so 

 far as they can be used as a guide, would seem to show that Dictyostroma, if on 

 further investigation it should prove to be a valid genus, is closely allied to Rosenella. 

 Possibly, if its minute structure were known, it might be found to embrace the 

 types which I have here referred to Rosenella, but on this point nothing certain 

 can be said at present. 



The coenosteum in Dictyostroma consists of very thick, undulating, concentri- 

 cally-disposed calcareous laminee, which are separated from one another by intervals 

 of about their own width (about two thirds of a millimetre), and. which give off 

 from their upper surface strong, remote, pointed radial pillars, which appear to 

 reach the under surface of the lamina next above that from which they spring, but 

 do not become amalgamated with it. The broken edges of the laminae, when seen 

 in vertical fractures, exhibit minute rounded apertures, but the precise nature of 

 these could only be determined by means of thin sections. 



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