Page:TransRoySocCanada 16 4 159-167.djvu/5

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Original curvature of carapace destroyed by pressure but a raised gastric region still indicated. The lateral margins are bordered by a depressed band which broadens towards the antero-lateral corners. Anteriorly a distinct V-shaped cervical furrow joins the two flat marginal bands and divides the carapace into two unequally sized areas. The anterior of these areas has three distinct spine-bearing ridges, a median one forming the base of the rostrum, which runs for half the distance to the cervical fold, and a pair of lateral ridges on either side, which are directed so as to converge when produced at a point near the forward end of the rostrum. These ridges abut on the cervical furrow but do not quite reach the anterior margin. The area behind the cervical furrow is marked by a very faint median keel which dies out entirely half a millimetre from the posterior border.

The surface viewed through a lens is roughly pitted, and the bases of larger spines lie on the rostrum and keels.

Dimensions: Extreme length 20 mm.; length, excluding rostrum, along median line, 13 mm.; length of rostrum projecting beyond anterior margin 5 mm.; width of rostrum at anterior margin 0.8 mm.; greatest width of carapace 17 mm.; apex of cervical furrow 7 mm. from the anterior border.

Locality: St. Rose mine, Inverness county, N.S. From roof of coal seam, associated with Naiadites, Ostracoda, and Palaeochara acadica.

Horizon: Coal Measures. (Lower Coal Measures?)

Remarks: Although the type spécimen of A. hillianus is proportionally narrower than the present one, the points of agreement are too many for varietal separation. Peach[1] has pointed out that in A. russellianus the serrations on the lateral margins are most probably the flattened spines that were borne along the whole length of lateral keels. As a result of the doublure of the test, and flattening accompanying fossilization, the serrations may appear to be restricted to the anterior portion of the carapace. This observation promotes caution in laying stress on the number of dentations visible on the margin, stated to be five on the type specimen of A. hillianus. The two strong spines figured by Dawson on either side of the base of the rostrum are seen in the present specimen to consist of two short spinous keels as present also in a similar position in A. russellianus and other species.

Dawson pointed out the close resemblance of A. hillianus to A. dubius (Prestwich). Yet the affinity must lie nearer A. grossarti Salter

  1. Peach, B. N., Monograph on the Higher Crustacea, Mem. Geol. Surv. G.B. Pal. Vol. I, No. 1, pp. 31-32, pl. iv. Figures 4, 6, 1908.