CYTHEROPTERON SPHENOIDES. 33 



CrTHEaE viEOiNEA, Joues. Sysfc. Lists, &c., Belfast Nat. Field Club, vol. i, 



Append, iii, 1875, pp. 81 aud 92. 

 — CONCENTRICA, var. viRGiNEA, Beiiss. Elbthalgcb., &c., 1874, p. 145, 



PI. I, fig. 14. Length '5 ; height '305 mm. 



PI. I, fig. 15. „ -85; „ -525 „ 



PL I, fig. 16. „ "7; thickness '45 mm. 



PI. I, fig. 17. height -55; „ '55 „ 



This form is very persistent in the Chalk of several localities, but the absence 

 of ornament seems to be its only distinction from C. concentricum. The faint 

 trace of reticulate structure in translucent valves, and some feeble ventral 

 wrinkles or riblets in one of the Irish specimens, strengthen its position as a 

 variety. 



M. Bosquet (op. cit., 1854, p. 81) has indicated that Reuss's G. Altlii may be the 

 same as the var. vvn/inea (1849), and Dr. Reuss in 1874 expressed his acquiescence 

 in this determination. The Lemberg specimen shows some ventral riblets. 



The recent Cytlieropteron Ixve, Brady and Norman, ' Trans. R. Dubl. Soc.,' 

 1889, p. 210, pi. XX, figs. 29 — 31, is remarkably similar, but is broader (higher) 

 in front, and quite destitute of ornament. 



Localities. — Chalh, Horstead, Gravesend, Magheramorne (Antrim), and Keady 

 Hill (Londonderry) ; Detritus, Charing ; Greensand, Cambridge and Warminster. 



Foreign. — Cretaceous, Grosau. 



2. Cytheropteron sphenoides (Beuss). Plate I, figs. 18 — 20. 



CxTHEEE SPHENOIDES, Beuss. Deukscli. Akad. "Wiss. "Wien, math.-nat. Class., 



vol. vii, 1854, p. 141, pi. xxvi, fig. 2. 

 — — — Elbthalgeb., &c., 1874, p. 147, pi. xxvii, fig. 7. 



Length "8 ; height '45 ; thickness "5 mm. 



Subtriangular, tumid, but depressed or pinched in at the anterior, dorsal, 

 and posterior margins. The thickness is greater and sudden just behind and 

 below the middle (figs. 19 and 20). The front end is obliquely rounded; the 

 ventral region is obliquely convex, with a thickened ridge above the flat ventral 

 area of the margin. The dorsal line slopes downwards and backwards, and the 

 hinge-teeth (fig. 19) are stronger than usual in the genus. The hinder end is 

 narrow, depressed, and serrate with a few denticles. 



In these features it differs from the Cretaceous and Tertiary C. triangulare 

 (Reuss), ' Suppl. Monogr. Tert. Ent.,' p. 44, pi. ii, figs. 19 ft — e, being more 

 convex centrally, and more depressed at the ends and back ; it is also more truly 



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