TJie Invertebrate Fauna of the Uitenhage Series. 161 



of a thorough comparison with known forms, or to warrant the use 

 of a new specific name. The posterior end is slightly broken away 

 so that the exact outline of the shell here cannot be clearly seen. 

 The length is considerably greater than the height ; the valves are 

 of a compressed form, most convex in the uppermost third, more 

 flattened below. Theumbones are little conspicuous and are directed 

 very slightly backwardly ; they are situated just posteriorly to the 

 middle of the shell. The upper margin slopes down very gently in 

 front of the umbo, with an almost straight profile which passes into 

 the evenly convex outline of the frontal margin. The posterior com- 

 pressed area is well defined, especially in the neighbourhood of the 

 umbo. 



Dimensions. 



Length 40 mm. 



Height, measured from the umbo 29 ,, 



Depth of one valve 8 ,, 



Occurrence. In the railway cutting between milestones 24-| 24f 

 on the line between Uitenhage and Graaff-Keinet, about three miles 

 from Uitenhage (331). 



Remarks. Comparison with English specimens of Thracia pliil- 

 lipsi Boem.,* from the Speeton Clay, shows that these differ widely 

 from the shell before us by their much more inflated and prominent 

 umbonal region and more steeply sloping antero-superior margin. 

 In general form and outline the African shell shows close simi- 

 larity to Thracia subtruncata Meek, from the Cretaceous strata 

 of Sucia Islands (British Columbia).! The outline also some- 

 what nearly resembles that of Thracia gracilis Meek and 

 Hayden,} from Cretaceous sandstone at the mouth of the Judith 

 River (on the Missouri), but the latter is rather more elongated 

 posteriorly. 



Another shell which may be brought into close comparison occurs 

 in the Aptian of Spain (Obon and Utrillas) and was described by 

 Coquand as Periploma lorieri. To judge by Coquand's figure, this 

 differs from the Uitenhage form by the steeper inclination of the 

 carinal ridge and the more nearly parallel direction of the upper and 

 lower margins anteriorly to the umbo. 



* Mya depressa Phillips, (1), Tab. 2, fig. 8 (non Sow.); Thracia phillipsiF. A. 

 Boemer, (2), p. 74, Taf. 10, fig. 1 (1841). 

 t Whiteaves (2), p. 140, pi. 17, fig. 7. 

 J Meek (2), p. 224, pi. 39, fig. 6. 

 Coquand (1), p. 100, pi. ix., figs. 5, 6. 



