202 DE. T. DAVIDSON ON EECEXT BEACHIOPODA. 



descriptions of Prof. Owen and Dr. S. P. Woodward. The cirri are of great length and 

 barbed throughout, with spine-like asperities ; in some cases they bifurcate near their 

 extremities and lie close together at theii- origin. In some specimens of Discinisca Icevis 

 great numbers of full-grown PedicelllncB, a genus of Bryozoa, adhered to the long barbed 

 cirri, looking like Llngulce with their long pliant peduncles. The smaller valve of 

 Discinisca atlantica was thus described by Dr. Gwyn JeflPreys {loc. cit. p. 252) from a 

 North- Atlantic specimen obtained during the ' Valorous ' Expedition : — " Plat, thin, 

 having near its middle a comparatively small round disk, within which is an oval slit 

 for the passage of the byssal (peduncle) of attachment ; this disk is slightly sunk within 

 any calcareous substance to which it is attached, as if the byssus had the power of 

 excavation ; the rest of the lower valve is free and concentrically striate, like the upper 

 valve : muscular (adductor) scars in the upper valve, club-shaped, rather close together ; 

 no scars observable in the lower valve. Not the slightest trace of tubular or perforated 

 structure could be detected in either valve witli one of Smith and Beck's best micro- 

 scopes, under a lens of one-fifth power." 



I have nothing further to state with reference to this species, which was fully described 

 in my ' Challenger ' Report. 



I am not certain that Discinisca atlantica has been hitherto positively found in the 

 fossil state ; but Dr. Gwyn Jeffreys thinks that the Dlscina fallax, S. Wood, from the 

 Crag of England, may perhaps be referable to the species under description. 



111. Discinisca Ctjmingii, Broderip, sp. (Plate XXVI. figs. 23-26.) 



Orbimla Cumingii, Broderip, Proc. Zool. Soc. p. 124, 1833, and Traus. Zool. Soc. vol. i. p. 143, 

 pi. xxiii. fig. 1, 1835. 



Orbicula strlgata, Broderip, Trans. Zool. Soc. vol. i. p. 143, pi. xxiii. fig. 1* 1833. 



Orbicula Cuminyi and 0. strlgata, G. B. Sowerby, Tlics. Coach, vol. i. p. 3G6, pi. Ixxiii. figs. 6, 7, 1846. 



Orbicula Cumingi, A. d'Orbigny, Voyage dans I'Ame'rique meridionals, vol. v. p. 677, 1847. 



Orbicula Cumingi and 0. strlgata, Davidson, Ann. & Mag. Nat. Hist. 2nd ser. vol. ix. p. 276, 1852. 



Dlscina Cumingi and 0. strlgata, E. Suess, Ueber die Wohnsitze der Bracliiopoden, Sitzungsb. k. Akad. 

 Wissensch. Wien, Bd. xxxvii. pp. 226-227, 1859. 



Orbicula Cumingi, L. Reeve, Monogr. of Orbicula, Conch. Icon. pi. i. fig. 6, 1862 (Mr. Reeve considers 

 0. strlgata, Broderip, to be a synonym). 



Dlscina Cumingi, Dall, Amer. Journ. of Couch, vol. vi. p. 77, 1870; Bull. ]\Ius. Comp. Zool. Harvard, 

 vol. iii. p. 42, 1871; and Proc. Acad. Nat. Sci. Philadelphia, p. 201, 1873. 



Shell orbicular, or elongated oval, broadest anteriorly. Larger or ujiper valve 

 moderately thick, limpet-like ; vertex submarginal ; surface covered with numerous very 

 fine (sometimes interrupted) raised striae, which radiate from the vertex to the margin of 

 the valve ; surface crossed likewise by numerous, fine, concentric lines of growth. 

 Lower valve extremely thin, concave or flat, often taking the shape of the object upon 

 which it rests. About half the posterior portion of the valve is taken up by an elongated 

 oval or heart-shaped, slightly concave disk or area, perforated longitudinally along the 

 middle by an oval-shaped fissure which does not reach either to the posterior margin of 

 the shell or to the anterior extremity of the depressed smooth disk. The remainder of 

 the surface of the valve is very finely striated radiately and crossed by numerous con- 



