352 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
This species bears a near resemblance to A. gracilis (St. J. and W.) ; its principal 
points of difference lie in the more angular form presented in transverse section, 
and the form of the internal cavity ; the posterior denticles are smaller, less frequent, 
and not so acutely pointed. 
Formation and locality : Mountain Limestone, Lowich, Northumberland. 
Ex coll. Woodwardian Museum, Cambridge. 
Acondylacanthus attenuatus, Davis. 
(Pl. XLVI, fig. 3.) 
Spine, much compressed, gracefully arched backwards, three inches in length, 
base imperfect, lowest portion °3 inch wide, gradually tapering towards the opposite 
extremity, and ending in a fine point. Anterior surface, with a prominent longitu- 
dinal keel, lateral surfaces with 12-14 longitudinal ridges, wider than the inter- 
mediate sulci; broad and flat at top, smooth ; towards the base the number of ridges 
increases by bifurcation. A slightly deeper and wider sulcus than the other, passes 
between the last posterior ridge and the latero-posterior angle. The angle is 
slightly rounded, and armed with a row of broadly implanted, recurved, sharply- 
pointed denticles, separated from each other by about their own diameter. 
Posterior surface hidden by the matrix. Transverse section at part preserved 
nearest the base, is two and a half times greater in diameter between the antero- 
posterior margins than between the lateral ones ; lateral surfaces converge anteriorly 
and form an acuminate apex ; posteriorly the surface curves inwards, forming on 
the surface a longitudinal median sulcus, with a row of denticles on each side. 
Internal cavity, small, oval, placed near the posterior margin, and extending not 
more than half the diameter of the spine towards the opposite one. 
This beautiful specimen appears to be unique. It was presented by the Rev. W. 
Stokes, with several others, to the Woodwardian Museum, at Cambridge. Tt 
presents a more graceful appearance than A. jenkinsoni, to which it bears some 
resemblance, but may be distinguished from it by the comparatively large size of 
the posterior denticles, and the form of its transverse section and internal cavity. 
From A, cole’, to which it also bears an outward resemblance, it is separated by 
the roundness and robustness of the spine of A. cole?, compared with the ovally 
lenticular, and compressed sides of A. attenuatus. 
This species offers several points of resemblance with the spine, Acondylacanthus 
(Leptacanthus) occidentalis, figured by Newberry and Worthen (‘Geol. Illinois,” 
Vol. I1., p. 116, pl. xu, fig. 2.) but appears to be sufficiently distinguished by the 
somewhat shallow posterior sulcus, and roundness and robustness of its denticles, 
those of A. occidentalis being conspicuously furrowed, bordered hy two rows of 
compressed and depressed hooks. 
Formation and locality : Carboniferous Limestone, Armagh. 
Ex coll, Woodwardian Museum, Cambridge. 
