FROM TANGANYIKA AND KENYA 67 



1883. Ostrea costata Sow. ; de Loriol : 77, pi. 11, figs. 8-17. 



1888. Ostrea (Alectryonia) costata Sow. ; Schlippe : 113, pi. 1, figs. 11, 12. 



1912. Alectryonia costata Sow. ; Lissajous : 65, pi. 8, figs. 19, 20. 



1916. Ostrea {Alectryonia) costata Sow. ; Jekelius : 230, pi. 4, figs. 3-6 ; pi. 6, fig. 9. 



1923. Alectryonia costata Sow. ; Cossmann : 4, pi. 5, figs. 5-8. 



1924. Ostrea costata Sow. ; Cossmann : 24, pi. 2, figs. 61-64. 

 1924. Ostrea (Alectryonia) costata Sow. ; Hennig : 33, pi. 3, fig. 2. 

 1929. Arctostrea costata (Sow.) ; Weir : 21, pi. 1, fig. 17. 



1933. Ostrea (Alectryonia) costata Sow. ; Ruiz, in Fabiani & Ruiz : 14, pi. 2, fig. 1. 

 19346. Lopha costata (Sow.) ; Arkell : 48, pi. 1, figs. 3-6. 

 1935. Lopha costata (Sow.) ; Cox : 173, pi. 17, fig. 13. 



Material. One specimen (no. LL. 35025) from the Toarcian and several from 

 later beds. 



Localities and horizons. Didimtu hill, 2 miles S. of Bur Mayo, N.E. Kenya ; 

 Upper Lias, Toarcian, Didimtu Beds. 3! miles W. of Melka Biini, N.E. Kenya ; 

 Callovian, Rukesa Shales. S. of Rahmu-Melka Murri road, 6 miles W. of Rahmu, 

 N.E. Kenya ; Callovian [?-Lower Oxfordian], Muddo Erri Limestones. 



Description. The Toarcian specimen now recorded is about 17 mm. high, with 

 a deep lower valve, the sides of which rise steeply from a rather large attachment 

 area. The sides have about 13 irregularly arranged costae, some of which have 

 arisen during growth by bifurcation of single costae, and which are prominent except 

 on the posterior and anterior ends of the valve. The costae are rounded at their 

 crests and are separated by deep but rounded intervals of about their own average 

 width. The upper valve is flat except for some irregularities and has a few weak 

 radial plications. The specimens from later formations call for no particular com- 

 ment. 



Remarks. I have hesitated before referring the Upper Liassic specimen to L. 

 costata, as typically this is a Bathonian species and records of its occurrence even as 

 early as the Bajocian have been queried (Whidborne 1883 : 492). Specimens from 

 the Bajocian of the Cotswolds which I would refer to the species are, however, in the 

 British Museum (Natural History). In typical specimens from the Great Oolite of 

 England, such as those figured by Morris & Lycett (1853) and by Arkell (1934&), the 

 plications are smaller and more numerous than in the specimen now recorded, and 

 this is also the case in European Bathonian specimens figured by Schlippe (1888) and 

 Cossmann (1923). In those figured by Lissajous (1912), however, the ribbing is of 

 about the same strength as in the present shell, and this is also the case in the English 

 Inferior Oolite specimens already mentioned. Cossmann (1924) has referred to the 

 variability of specimens of L. costata from the French Callovian, and has stated that 

 the number of ribs ranges from 12 to 18 irrespective of the geological horizon. L. 

 costata is here accepted as a species ranging from Toarcian to Callovian, the present 

 being the first record of its occurrence in the former stage. Thevenin (19086 : 21, 

 pi. 4, figs. 10, iofl) has recorded a small plicated oyster from the Upper Lias of 

 Madagascar under the name Ostrea subserrata Goldfuss, although Goldfuss's species 

 is now known to have been a Plicatula. The specimen from Madagascar has narrow- 



