FoniminiJ'cra and Odracoda. 235 



ventral margin hollowed posteriorly ; dorsal margin sinuous. Sur- 

 face of the valve sloping gently from behind to the anterior border 

 and depressed or even sulcated near the antero-dorsal angle. 

 Towards the ventral margin the valve is steep and sometimes 

 angulated and sloping away to the dorsal. Highest in the middle of 

 the posterior third. Hinge showing the characteristic crenulated 

 crests and grooves of this genus. Surface of valve marked with 

 numerous impressed puncta, having a quasi-linear arrangement. 

 Length of left valve 1-16 mm. ; height -66 mm. 

 Length of right valve of a larger specimen 1-3 mm. ; height 

 •83 mm. 



The shape of the carapace in this species may be compared to an 

 extremely attenuated form of Cythcridea perforata (Romer).* The 

 tumid form of the posterior portion makes its resemblance to 

 Cjjtheroptcron sphenoides (Reuss) \ very close, but the hingement is 

 different and the ventral margin hardly high enough. In addition 

 to the evidence of the hinge characters of Cytheridea, our specimens 

 have the impressed puncta seen also in Cythcridea perforata. 



Genus CYTHEROPTERON G. 0. Sars. 



Cytheropteron elongato-concentricum sp. nov. 

 Plate XXIX., ligs. 23, 23rt. 



Specific Characters. — Valves subrhomboidal, elongate, with a 

 broad posterior flange, and one less strongly marked, anteriorly. 

 Ventral border steep and well rounded. Surface tumid, and highest 

 just behind the middle of the valve. Surface ornamented with poly- 

 gonal excavations, not so numerous nor so distinctly concentric as 

 in G. concentricum (Eeuss),| with which this present specimen 

 seems most nearly allied. 



Length '66 mm. ; height '33 mm. ; thickness "16 mm. 



The elongate carapace, the conspicuous posterior flange, and the 

 comparatively coarse pittings serve to separate this from the well- 

 known and widely distributed Cytiicroptcron concentricum of 

 Europe. 



* See Jones and Hinde, 1890, Mon. Cret. Entom., p. 29, pi. i., figs. 1-4. 

 t See Jones and Hinde, 1890, Mon. Cret. Entom., p. 33, pi. i., figs. 18-20. 

 + See Jones and Hinde, 1890, Mon. Cret. Entom., p. 31, pi. i., figs. 5-10 ; pi. iv., 

 fig.l9. 



