72 BRITISH SILURIAN BRACHIOPODA. 
examination and comparison of the English shell so named with the original example 
of the Canadian Ordicula cancellata, Sowerby,’ which had been presented by Dr. 
J. J. Bigsby to that Museum, Mr. Etheridge and myself arrived at the conclusion 
that they were specifically distinct, and that a separate designation should be given to 
the English shell. D. Siluriana is comparatively broader, the apex sub-marginal, 
that is to say, more elevated and distinct from the margin-line of the shell, while in 
D. cancellata there exists no distinct apex, the free valve being orbicular and very 
slightly convex, the posterior portion forming a small, pointed, incurved beak. The 
sculpture is also slightly different in its details, for the Canadian shell is more regularly 
reticulated, on account of the concentric raised lines dividing the cells being more 
continuous and regular, while in D. Si/uriana there exists no continuous concentric lines, 
as has been already described. 
Position and Locality. In the Caradoc beds of Horderley, Salop ; also at Marshbrook, 
in the same formation. (Mus. Pract. Geology.) 
Sub-genus—OrBicuLowEa, @’Orb., 1847. 
OrBIcULA and Disctna, in part, of some authors; Schizotreta, Kutorga ;? Davidson, 
Monogr., vol. i, Introduction, p. 129. 
Prof. Morris justly observes in his interesting ‘ Note on the Genus Siphonotreta,’® that 
Dr.S. Kutorga’s genus Schzzotreta is synonymous with Orbiculoidea, D’ Orbigny, and presents 
1 Mr. Sowerby gives us the following description of his shell, Orbceula cancellata, G. B. Sowerby, 
‘Zoological Journal,’ vol. ii, pl. xi, fig. 6, 1826:—‘Sp. O. cancellata, testa orbiculari, vertice postico, 
marginali; valvarum superficie lineis elevatis, confertis, radiantibus, lineis incrementi elevatis decussatis ; 
valvee inferioris vertice excentrali, levi, depresso, sinu byssi parvo, brevi. The general form of this shell 
is orbicular and very flat, being more gibbous near the posterior extremity ; the vertex of the upper valve is 
quite marginal and posterior ; its surface is covered with close-set elevated lines, radiating from the vertex, 
and which are crossed by the elevated lines of growth, so that the entire surface has a finely reticulated 
appearance ; the vertex of the lower valve is also nearly marginal, having at the posterior edge a rather deep 
cavity, in which the sinus (through which the disc of attachment passes) is placed ; the surface of this valve 
is reticulated in the same manner as in the other, except near the umbo, where it is smooth, and the lines 
of growth are not elevated, but form complete rings, partly descending into the cavity above mentioned ; the 
shell is extremely thin ; it occurs in a light brownish-grey limestone, containing also remains of Terebratule 
and Coralloids. I am indebted to Dr. Bigsby for the opportunity of describing this species, which he brought 
from horizontal beds of limestone, resting on augitic trap, one mile north of Montreal, in Lower Canada.” In 
the ‘Geology of Canada,’ “ Palzeozoic Fossils,” vol. i, p. 52, 1862, Mr. Billings describes two more Canadian 
Discine, which appear to resemble D. cancellata, namely, Trematis Hawaensis and T. Huronensis. 
2 «Ueber die Siphonotreteze, Verhandlungen der Kaiserlichen Mineralogischen Gesellschaft fiir das 
Jahr, 1847, p. 250, St. Petersburgh, 1848. 
3 «Annals and Mag. of Nat. Hist.,’ 2nd ser. vol. iv, p. 315, Nov., 1849. 
