R. BuUen Newton — Egyjitian Cretaceous Shells. 397 



Dimensions, 



Largest specimen (both valves) — Height ... 103 mm. 



Length 73 ,, 



Diameter ... ... 68 ,, 



This species has certain resemblances to 0. dicliotoma, Bayle, an 

 Algerian shell, but differs from it in being curved or arched and 

 possessing a rough spiny exterior. It may be compared also with 

 another Algerian species, 0. syphax, which, however, has more or 

 less exogyriform umbones, especially during the earlier period of its 

 development. 



Eemarks. — It is interesting to note that M. Peron, in his work 

 on the Cretaceous Mollusca of Tunis, has united the two species 

 0. dicliotoma and 0. acanthonota, but for present purposes it seems 

 desirable to regard them as separate forms. The Egyptian examples 

 of this species are in a splendid state of preservation and consist of 

 both old and young shells. The matrix is a soft white limestone, 

 resembling chalk. 



HoKizoN. — Turonian. 



DiSTKiBUTioN. — Algeria ; Golea, west of Abu Eoasch ; west of 

 the Gizeh Pyramids : Coll. Geol. Surv. Egypt (No. 64, Box No. 31c). 



OsTREA Lyonsi, sp. nov. PI. XV, Figs. 5-7. 

 Description. — Species oblong, oval, small, and of variable 

 dimensions ; lower valve convex, lamellose, and prominently plicated ; 

 ligamented area narrow, small, triangular, and rather oblique ; 

 muscular scar antero-ventral, serailunate, and concentrically striated ; 

 upper valve depressed, possessing moderately deep marginal plica- 

 tions, and ornamented with concentric growth-lines. 

 Dimensions. 



Largest lower valve — Height... ... ... ... 40 mm. 



Length 30 ,, 



Diameter ... ... ... 14 ,, 



This is a very distinctive form of Ostrea, which appears to differ 

 in many details from other species. The lower valve is often much 

 inflated at the umbones, where it has a steep flattened adherent 

 surface; the plications are rounded, elevated, few (four or five), and 

 most prominent at the ventral margin. The valves, according to 

 age, difi'er a good deal in relative convexity, many being depressed. 

 The species is related to 0. Nicmsei, Coquand, from the Cretaceous 

 of Algeria, which is of a similarly lamellose structure and having 

 the same kind of widely distant plications, but differing in its flatter 

 and more equally convex valves, as well as in its generally more 

 circular shape. 



Eemarks. — From the number of specimens sent for determination, 

 this species may be regarded as fairly abundant. The shells, largely 

 mineralized with incrustations of " Beekite," are found in a grey 

 compact limestone associated with Arctica Barroisi, as well as in 

 a softer limestone of a fawn colour. 



Horizon. — Turonian. 



Distribution. — Hills west of Jebel Zait : Coll. Geol. Surv. Egypt 

 (No. 637, Box No. 48a). 



