642 MR. E. B. NEWTON ON [NOV. I9II, 



Pelecypoda. 



OSTREA Cf. VENTILABRTJM GoldfllSS. (PI. XLV, figS. 9-11.) 



Ostrea ventilabrum Goldfuss, ' Petrefacta Germanise ' 1833, vol. ii, pi. lxxvi, 



figs. 4-4 c & p. 13. 

 Ostrea prona S. V. Wood= ventilabrum S. V. Wood, Monogr. Pal. Soc. 1861 & 



1871, pi. iii, fig. 3 & pp. 29, 181 (Index). [Eocene Mollusca.] 

 Ostrea ventilabrum A. von Kcenen, ' Norddeutsche Unter-Oligocan Mollusken- 



Fauna ' Abhandl. Geol. Specialkarte Preussen, 1893, vol. x, pt. 5, pi. lxiv, 



figs. 5-8 & p. 1011; Rovereto, Atti R. Univ. Genova, 1900, vol. xv, p. 48; 



Blanckenhorn, Zeitschr. Deutsch. Geol. Gesellsch. 1900, vol. Iii, p. 459. 



lie marks. — This form is represented by a number of valves, 

 the lower being of cresceutic contour while the upper is more or 

 less oblong. The lower valves are also fairly thick and arched, 

 sometimes more spreading and depressed, and ornamented with 

 numerous regular stout and radiating costse which in their descent 

 from the umbonal area are often dichotomized. The periodical 

 growth-lines of the upper valve consist of irregularly spaced 

 concentric ridges, the central area of the same valve being well 

 elevated above a somewhat depressed ventral region. Both valves 

 exhibit a slightly concave or nearly straight anterior margin. 

 The umbones show a forward inclination, bat are not exogyri- 

 form ; while the adductor-sear impression is extensive and antero- 

 ventral. 



Dimensions in millimetres (of adult valves belonging to 

 different individuals) : — 



Lower valve. Upper valve. 



Length 35 40 



Height 53 65 (approximate). 



Diameter 25 15 



Among oysters exhibiting similar relationships may be men- 

 tioned 0. jimbrioides of Eolle, from the Austrian Oligocene, 1 

 0. cyathula of Lamarck, var. fimhriata Hoernes, which Mayer- 

 Eymar doubtfully recognized from fragments in the Lower Oligo- 

 cene (Tongrian) of Egypt, 2 besides 0. ventilabrum and 0. fimbriate^ 

 both scheduled by Dr. Blanckenhorn from similar deposits of the 

 same country. The true 0. ventilabrum is therefore characteristic 

 of Lower Oligocene deposits, and is known from England, Germany, 

 Austria, Northern Italy, and Egypt. 



Occurrence. — The valves are considerably worn, and mostly 

 associated with a cream-coloured limestone weathering to a reddish 

 colour, containing numerous Nummulites. 



Localities. — South-west of Merj ; east of the shrine of Sidi 

 Mahomet XLahridi, east of Slonta ; Old Cistern, east of Slonta ; 

 north of Slonta ; Roman Fort, north-west of Slonta. 



1 Sitz. K. Akad. Wissensch. Wien, vol. xxxv (1859) p. 204 & pi. ii, figs. 1-3 c. 



2 Bull. Inst. Egyptien, 1896, ser. 3, no. 6, p. 94. 



