182 CONTRIBUTIONS TO CANADIAN PALEONTOLOGY. 



On emerging from the central pit the septa become augmented in num- 

 ber, many of them bifurcating or trifurcating, and continue outward as 

 superficial radial foldings of the calycinal margins. A double row of 

 minute pore-openings occurs in each septal ridge, one row on either side 

 of the central line, the openings in either row generally alternating with 

 those of the other. In vertical sections each corallite is seen to be com- 

 posed of a series of invaginated calyces whose margins, uniting with those 

 of adjacent calyces at the same level, form a succession of superposed 

 continuous floors. In the interseptal spaces in the vicinity of the central 

 pit occur small arched dissepiments rising obliquely outward ; these con- 

 tinued outward, in the area to which the septa do not extend as vertical 

 lamella?, become larger and more unequal in size, forming vesicular tissue, 

 on which the successive calycinal floors rest. Small inconspicuous, hori- 

 zontal tabula 1 occupy the centre of the visceral chamber. Increase by 

 intercalicular gemmation. 



Locality. — Grand Manitoulin Island, Lake Huron, collected by R. 

 Bell and H. G. Vennor, in 1865, and by J. Townsend, in 1882 and 1883 ; 

 Niagara formation. 



This species cannot be distinguished from A. striatum, d'Orbigny, 

 according to Rominger, who (on page 131, op. cit.) says " it is impossible 

 to draw a dividing line between them ; all gradations of sizes, from the 

 large to the small forms, can be found associated in the same localities, 

 and in structure not the least difference exists between them." 



Arachnophyllum mamillare. Dale Owen. (Sp.) 

 Plate XV., fig. 4. 



A*tr<a mamillaris, Dale Owen. 1844. Rep. Geol. Explor. Iowa, Wisconsin and Illinois, 



p. 70, pi. XIV., fig. 3. 

 Strombodes mamillatus, Rominger. 1876. Geol. Sur. Mich., Foss. Corals, p. 132, pi. 



XLVIIL, fig. 4. 



To this species is assigned a specimen collected at Grand Manitoulin 

 Island, Lake Huron, by J. Townsend, in 1883 ; it agrees in the size and 

 shape of its calyces with the figure of the type specimen (op. cit., pi. xiv- 

 fig. 3) and with Rominger 's description and figure of specimens from the 

 Niagara rocks of Point Detour, Lake Huron. 



The Grand Manitoulin Island specimen may be described as follows : — 



Corallum discoidal, somewhat convex above, flat below, measuring 15 



cent, across and 4 cent, high at the centre. Basal surface wrinkled, 



covered by an epitheca showing concentric growth lines, and with a small 



central point of attachment. Corallites slightly divergent, opening on 



