CANADIAN PALAEOZOIC CORALS. 181 



expanded margins uniting with those of adjacent corallites form a suc- 

 cession of continuous floors, supported by coarse vesicular structure. 

 Calyces shallow, impressed centrally with a small pit that is at times 

 surrounded by an elevated rim ; their marginal boundaries more or less 

 distinct and polygonal in outline, sometimes scarcely defined. Septa 

 lamellar and continuous vertically in the neighbourhood of the central 

 pit, outwardly developing into superficial radial foldings of the calycinal 

 margins. Tabuke generally inconspicuous, forming a narrow axial area 

 at the centre of the visceral chamber. Dissepiments occupying the 

 interseptal loculi in a zone surrounding the tabulae ; in the peripheral 

 area they develop into blister-like plates that form the supporting 

 vesicular tissue continuous from one corallite into the next. Septa 

 radii at times poriferous. 



Type species. — A. (Acervularia) Baltica, Lonsdale. 



Range. — Silurian. 



Arachnophyllum pentagonum, Goldfuss. (Sp.) 

 Plate XV., figs. 3, 3a. 



Strombodcs pentagon as, Goldfuss. 1826. Petrefacta Germanic, vol. I., p. 62, pi. XXI., 

 figs. 2a, 6. 

 „ Milne- Ed wards and Haime. 1851. Polyp. Foss. des Terr. 



Palaeoz., p. 430. 



Rominger. 1876. Geol. Sur. Mich., Foss. Corals, p. 130, pi. 

 XLVIIL, figs. 1 and 2. 



Whitfield. 1882. Geol. of Wisconsin, vol. IV., p. 275, pi. XV., 

 fig. 5. 



Corallum compound, explanate, discoidal, slightly convex above, with 

 a flat or undulating lower surface roughened by irregular coarse wrinkles 

 of growth, and with a small central base of attachment ; attaining a 

 diameter of over 13 cent, and a thichness of 3 or 4 cent. Epitheca 

 covering the basal surface, marked by fine concentric growth lines. Poly- 

 paria upright, very slightly di T ergent, confluent, opening above in shallow 

 calyces, impressed at the centre with a small pit, and meeting in obtusely 

 elevated polygonal outlines. Calyces from about 1 to over 2 cent, in 

 width, radially marked by fine, obtusely angular septal ridges that con- 

 nect with those of adjacent calyces. Central pit from about 2 • 5 to 6 mm. 

 wide, or averaging about one-fourth the diameter of the calyces. Septa 

 numbering from thirty to forty at the margin of the central pit, within which 

 they are lamellar and continuous vertically ; of these half reach the centre 

 of the visceral chamber, whilst the remainder stop short of the centre, 

 frequently all or some of the septa coalesce in sets of twos near the bot- 

 tom of the pit, each pair continuing to the centre as a single septum. 



