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



ClatJirodictyon regulare, Rosen, sp. 

 (PI. il. figs. 5 and 6.) 



Stromatopora reqularis, von Rosen, Ueber die Natur der Stromatoporen, 

 p. 74, pi. ix. 6gs. 1-4 (1867). 



Ccenosteum of small size ; sometimes laminar and discoidal, 

 with a wrinkled basal epitheca ; sometimes incrusting foreign 

 bodies. The largest specimen examined is less than 2 inches 

 in diameter, with a maximum thickness of half an inch. The 

 surface is smooth or slightly undulated without " mamelons," 

 and often showing the edges of the exfoliated laminae. Astro- 

 rhizEe are apparently absent. In well-preserved examples the 

 surface is studded with small rounded tubercles, representing 

 the free ends of the radial pillars, which often send out radia- 

 ting horizontal prolongations, inclosingminute interstitial pores. 



As regards internal structure, the skeleton is made up of 

 horizontal or slightly flexuous concentric laminaj, of which 

 about six occupy the space of 1 millim. The laminee are 

 thick, often with a median dark line, and sliglitly crumpled 

 (PI. II. fig. 5). At each point of inflection the lamina sends 

 down from its under surface a stout radial pillar, which may 

 only project a short way into the interlaminar space, but 

 which, more commonly, becomes connected with the lamina 

 next below. The interlaminar spaces thus become broken up 

 into rows of regular oblong cells, which are about i millim. 

 in length, but vary much in this respect. Tangential sections 

 exhibit large, rounded, dark dots, representing the cut ends 

 of the radial pillars (PL II. fig. 6). These are often con- 

 nected together by distinct radiating " arms," thus showing 

 an imperfect form of the " hexactinellid " structure so cha- 

 racteristic of the species of the genus Actmostroma. Astro- 

 rhizal canals do not appear to be developed. 



Obs. This species is readily recognized by its slightly 

 inflected, thick laminge, its stout radial pillars, the oblong form 

 of the regularly disposed interlaminar cells as seen in vertical 

 sections, and the presence of radiating " arms " connecting 

 the radial pillars. In its general characters, both external 

 and internal, it most closely resembles C. striatellum, d'Orb. ; 

 but it is an altogether smaller form, its skeleton is much 

 finer, and the form of the radial pillars and interlaminar cells 

 is quite diff'erent. 



Formation and Locality. Wenlock Limestone, Dudley 

 (rare). It occurs also in the Wenlock Limestone of Wisby, 

 Gotland. Von Rosen's original specimen (which I have 

 examined) is from the Silurian (" Zone of Pentamerus estho- 

 nus ") of Kleine-Ruhde, Esthonia. 



