i-> JURASSIC BIVALVIA AND GASTROPODA 



the present work as Cirrus mazerasensis sp. no v. Unfortunately, this material is in- 

 sufficient to establish the geological age of the sample, but it is improbable that it is 

 pre- Jurassic in view of the rareness of Cirrus in rocks older than the Lias. It is 

 suggested that a Bajocian age may be assigned to the sample until further evidence 

 is forthcoming. 



Bajocian depcsits occur along the Tanganyika Central Railway between Ngeren- 

 gere in the west and a point between Kidugallo and Magindu in the east, but it is 

 still uncertain where to draw their upper limit. They also crop out in the area to 

 the north and south. The geology of this district was described by Hennig, who 

 distinguished (1924 : 114, 121) between the Ngerengere Beds, continental deposits 

 belonging to the Karroo System and thought by him to be Liassic in age, and the 

 Ruvu Beds, which he considered to range from the Aalenian to the Oxfordian. No 

 specimens from "Reek's fossil bed" (see p. 8), thought to be the lowest fossiliferous 

 horizon of the local Jurassic series, have been examined in the course of the present 

 work. That the Lower Bajocian (Aalenian) is represented in this area is shown by 

 Arkell's (1956 : 330) record of ammonites of this age in carboniferous shales en- 

 countered in boreholes in search of limestone north of Kidugallo. Some bedding 

 planes of these shales are covered with specimens of the bivalve Bositra buchi 

 (Roemer) [ = Posidonia ornati Quenstedt]. The " Posidonia " from Kissemo, N. of 

 Kidugallo, recorded by Hennig (1924 : 43), may have come from about the same 

 horizon. The Kidugallo Oolite, a formation overlying " Reek's fossil bed " and also 

 included by Hennig in his Lower Ruvu Beds, yielded a number of Pectinidae and 

 other molluscs recorded by Hennig (1924 : 14-20), but no fossils from this horizon 

 have been seen by the present writer. Some molluscs now described came, however, 

 from two small quarries north of Ngerengere Station, where the horizon is close to 

 the junction of gneiss and sediment and probably fairly low in the local succession of 

 fossiliferous Jurassic rocks. The bivalves include the species Eomiodon baroni 

 (Newton) and Bakevellia iraonensis (Newton), both originally described from Mada- 

 gascar, the former from the Bathonian, the latter from Middle Jurassic beds the 

 precise age of which has not hitherto been recorded. 



Hennig's Middle Ruvu Beds and the Station Beds of King (1954 : 15) are approxi- 

 mately synonymous. Aitken (in Quennell ct al., 1956 : 180-181) has compiled a 

 list of their fossils as recorded by Hennig. The majority are probably of Bathonian 

 age, but those from the more easterly localities (Hennig 1924 : 50-55) may be from 

 the Bathonian. Of the molluscs described in the present work, it is most probable 

 that, in addition to those from near Ngerengere, specimens localized as Kidugallo 

 and as 6 miles N.W., 5 miles N.W., 2$ miles N.N.W., i| miles N.N.W., and i\ miles 

 E. of that place are all of Bajocian age. The full list of Bajocian species definitely 

 identified from this area in the course of the present work is, therefore, as follows : 



Modiolus anatiniis (Smith) 



Bositra buchi (Roemer) 



Bakevellia iraonensis (Newton) 



Lopha gregarea (J. Sowerby) 



Trigonia costata Parkinson 



