Vol. 52.] FROM THE CARBONIFEROUS LIMESTONE. 445 



specimens discovered by Mr. Spencer G. Perceval l from the Lower 

 Limestone Shales of Combe Down, Henbury, near Bristol, have 

 lately been acquired by the British Museum (Nat. Hist.), but they 

 are also partially silicified and do not show the structure so clearly 

 as the American examples. The colony is small, wedge-shaped, 

 with a compressed base which is quite free ; there are from two to 

 four corallites arranged in a single lateral series. (PI. XXIII. 

 figs. 16 & 16 a.) In some cases the upper margins of the corallites 

 project beyond the general surface, and are free, sharp-edged, and 

 minutely crenulate. The outer surface of the corallum has longi- 

 tudinal, subparallel, slightly wavy ridges, and in the furrows between 

 these are rows of mural pores, which extend directly through the 

 wall and open into the interior of the calices. The calices are 

 conical, varying from nearly circular to oval or elliptical in section ; 

 within they show the rows of pores, and between these are faintly- 

 marked ridges or lines, which may represent septa. Pores or canals 

 also pass between and connect the corallites. The wall in the 

 compressed basal portion of the corallum is now, in the specimens 

 examined, solidly replaced by silica, and its original structure 

 doubtful, but there are no indications of a vermiculate ecenenchyma, 

 and it may have been perforated by simple pores or straight canals, 

 like those in the upper portion of the corallum, which, moreover, 

 closely resemble those already described in P. humilis. 



Turning now to Hydnopora (?) cyclostoma, Phill., of which 

 examples are fairly common in the Carboniferous Limestone of 

 Northumberland, and of Fifeshire and other places in Scotland, 

 we find it growing in small colonies usually of from two to six 

 corallites, but in some cases twelve have been observed. The 

 corallum is invariably attached to some foreign body, and its basal 

 portion in part conforms to the figure of the shell or other organism 

 to which it is affixed, and in part is free and covered with con- 

 centric wrinkles or ridges. The calices are open, with nearly 

 straight sides ; their margins may be either free or on a level with 

 adjoining calices ; and the interiors are furnished with numerous 

 tubercles or blunt spines, which are crowded over the bottom of the 

 cup and disposed regularly in rows on the sides, where they appa- 

 rently represent the septa. The outer surface of the coral has 

 interrupted sinuous ridges and granules, with, in places, irregular 

 apertures between. The structure of the wall, as shown in sections, 

 is of a remarkable character. The basal portion or floor within 

 each calice, and the sides as well, nearly if not quite up to the 

 margin, consists of a layer of solid, non-perforate, calcareous tissue, 

 outside of which, occupying the space between the calices and also 

 forming the basal layer of the corallum, there is a well-marked 

 layer of an openly porous or lacunar reticulate tissue, strikingly 

 similar to that of recent perforate corals. This layer, which may 

 be considered as a porous ecenenchyma, has been very carefully 

 described and figured by Etheridge and Nicholson.' 2 



The minute structure in H. (?) cyclostoma is of radiate crystalline 



1 Geol. Mag. 1876, p. 267. 



2 Ann. & Mag. Nat. Hist. ser. 5, vol. i. (1878) p. 215, pi. xii. figs. 7 & 8. 



