276 BULLETIN OF THE NATURAL HISTORY SOCIETY. 
near this furrow, which perhaps indicate the position of the central 
muscles ; the posterior adductors are indicated by bosses on the mould 
near the cardinal line, and the lateral muscles by depressions near the 
ends of the cardinal area. Fine radiating vascular lines are visible in 
the front half of the valve on each side of the median ridge. In 
young valves this ridge is only two-thirds of the length of that in the 
adult valve, the anterior third being smooth. 
Sculpture . — The surface is marked by minute concentric beaded 
ridges, visible only with a strong lens ; there are stronger growth lines 
at intervals. 
Size . — Length, 3 mm. Width, 3^ mm. Height of the ventral 
valve, one-fifth to one-quarter less. 
Horizon and locality . — The fine dark grey shales of the Dictyonema 
beds (C. 3c) at McLeod Brook, Cape Breton. 
On re-examining the specimens from this horizon at Navy Island, 
St. John, N. B., which I had compared with A. Baileyi, of the Para- 
doxides beds, I find it is identical with the species from McLeod brook. 
It is distinguished from A. Baileyi by the long, sharp median ridge of 
the dorsal valve ; the convex summit of the mould of the ventral 
valve also distinguishes it from that species, in which the summit is 
concave, and proportionately smaller. 
From A. socialis, Von Seebeck,* this species is distinguished by its 
somewhat larger size, and by the absence of the sharp wedge-shaped 
furrow in the top of the mould of the ventral valve ; also by the 
absence of the strong lateral furrows in the mould of the dorsal valve 
of that species ; also by the deeper and longer mesian furrow of the 
dorsal valve of the McLeod brook species. 
From A. yemma, Walcott,! this species is distinct by the convex 
top of the mould of the ventral valve, by the absence of an area to 
the dorsal valve, and the enlarged posterior end of the median ridge 
in this valve. 
From A. gemmula\ this species is distinct by its larger size, convex 
summit of the ventral mould ; and by the smaller scars of the posterior 
adductors and sharper and longer median ridge of the dorsal valve. 
* Brachiopcda of the Paradoxides beds[of Sweden. G. Linnarsson Stoikhc In. 1876. p. 16 
pi. iii, fiiiS. 32-35. 
t U. S. Geol Surv. Bull., No. 30. p. 98, p 1 . viii, figs. la-6. 
X Roy. Soc Can. Trans., vol xi, sec. iv, p. 87, pi xvi, fig. 2a-d. 
