20 Proi. H. A. Nicholson on some new or 



lenticnlar vesicles of which the entire ccenosteum is composed, 

 and which bear on their upper surfaces prominent, pointed, 

 and close-set tubercles (PL I. fig. 4). Tangential sections 

 (PI. I. fig. 5) exhibit the transversely-divided ends of the 

 tubercles or the irregularly- cut edges of the vesicles. 



The species is structurally very similar to Rosenella macro- 

 cystis, Nich., from the Wenlock Limestone of Gotland, but 

 is separated by the much smaller size of the vesicles. I have 

 examined the original specimens upon which von Rosen 

 founded the species, and I have also collected others myself; 

 but I have never seen any example satisfactorily exhibiting 

 the form of the ccenosteum or its mode of growth. The 

 species was, however, evidently of large size and apparently 

 non-encrusting. 



Formation and Localitij. " Zone of Pentamerus estJionus " 

 (Silurian), Kattentack, Esthonia. Rosen's type specimen is 

 from St. Johannis, in Oesel. 



Rosenella macrocystis^ Nich. (PI. I. fig. 8.) 



Rose7iella macrocystts, Nicholson, Mon. Brit. Strom, p. 84, pi. vii. 

 figs. 12 and 13 (1885). (Figured without description.) 



Ccenosteum laminar, with a basal peduncle of attachment 

 and a concentrically-wrinkled epitheca. Surface flat, without 

 " maraelons," showing no astrorhizal grooves, and covered 

 uniformly with miuute, extremely close-set granules or 

 tubercles. The ccenosteum is composed of approximately 

 horizontal plates, which are so undulated or bent as to give 

 rise to a tissue of elongated and greatly flattened vesicles of 

 variable sizes, the larger ones being commonly from 5 to 15 

 millim. in length. Each lamina has its uj^per surface 

 covered with close-set minute tubercles, which fall short of 

 the lamina next above. Tangential sections show the cut 

 ends of the tubercles or the irregularly divided edges of the 

 undulating laminse. 



Ohs. In general structure this species closely resembles 

 U. dentata, Rosen. It is, however, sufficiently distinguished 

 from this by the much more minute size of the tubercles 

 which cover the upper surfaces of the laminse, and by the 

 much larger size of the vesicles which make up the whole 

 ccenosteum. It is, moreover, a laminar form with an epi- 

 theca, whereas R. dentata would rather appear to have grown 

 in large masses. 



I have only seen a single specimen of R. macrocystisy 

 which was collected in Gotland by Dr. George J. Hinde, who 

 was good enough to submit it to me for examination. la 



