4 Prof. H. A. Nicholson on some new or 



therefore much more nearly quadrangular than is the case in 

 C. vesiculosum. The form which I have named G. crassiim 

 is sufficiently distinguished by the comparatively small num- 

 ber of laminas in a given space and by the great thickness of 

 the skeleton-fibre. Lastly, C. fast /'(/iatum is snf^c'iently sepa- 

 rated from the present species by the much greater width of 

 the interlaniinar spaces and the chevron-like folding of the 

 concentric laminaj. 



Formation and Locality. So far as known, the species is 

 wholly Silurian (Upper Silurian). The type-specimen is 

 from the Clinton formation of Yellowsprings, Ohio. It 

 occurs also in the Niagara Limestone at Thorold, Ontario, 

 and probably at many other localities in this formation. In 

 the Wenlock Limestone of Britain it occurs at Mucli Wenlock, 

 Dudley, L-onbridge, Dorinington, &c. I have also collected 

 specimens apparently belonging to this form from the Silurian 

 (" Zone of Fentamerus esthonus^'') of Kattentack, Esthonia, 

 and from the " Raikiill Beds " of Eaikiill. In Scotland it 

 has been collected by Mrs. Robert Gray in the Silurian rocks 

 of Woodland Point, near Girvan. 



Clatlirodictyon variolare^ Rosen, sp. 

 (PL I. figs. 4-6.) 



Stromatopora variolaris, von Rosen, Uebsr die Natur der Stromato- 

 poren, p. Gl, pi. ii. figs. 2-5 (1867J. 



Coenosteum laminar, hemispherical, or massive, with a con- 

 centrically-wrinkled basal epitheca, and often attaining a large 

 size. The surface may be smooth or may exhibit numerous 

 " mamelons/' which are sometimes small and pointed or, more 

 commonly, low and rounded (PI. I. fig. 4). Astrorhizfe are 

 present, but are of small size, and do not appear to correspond 

 in any special way with the surface-eminences or " mamelons," 

 when these latter structures are present. 



The internal structure is very similar to that of C. vesicu- 

 losum, the skeleton being composed of close-set concentric 

 lamina?, which are horizontal or only slightly undulated, and 

 xire minutely crumpled, so as to give rise, along with the 

 incomplete radial pillars, to a fine vesicular tissue. From 

 eight to ten laminte occupy a space of 1 millim. ; but the 

 interlaminar spaces are of unequal sizes, rows of large vesicles 

 alternating with rows of small ones (PI. I. fig. 5). In general, 

 rows of large vesicles are separated by two or three rows of 

 much smaller cells ; but there may be only a single row of 

 the latter or they may even be wanting in places. Tangen- 



