190 RALPH TATE. 
like depression beneath the umbo, the anterior projection narrow 
lanceolate. The medial shell-layer is pearly. Umbo-ventral 
diameter 33, antero-posterior diameter 58, anterior radius 26, 
posterior radius 32. 
The example of this fossil analogue of Thracia Watsoni, which 
has served for the foregoing measurements, has approximately the 
same length and breadth as it; however the fossil is more inequi- 
lateral as indicated by the following measures of the anterior and 
posterior radii, in 7. Watsoni they are 28-5 and 29-5, in 
P. anatineformis they are 32 and 26; moreover the fossil is 
more attenuated posteriorly and the plications are slender. It is 
noteworthy that the successional type of this genus occupies the 
same area as its ancestor and in an intermediate latitudinal 
position. 
Genus Myochama. 
IT am not aware if this genus, peculiar to Australia, is known 
in a fossil state, the discovery therefore of species in our tertiary 
deposits is of interest. The fossil representatives are I. rugata 
in the Eocene of Spring Creek and Gellibrand River, and IL. plana 
in the Miocene of Reeve’s River, Gippsland Lakes, and possibly 
the same species in the Older Pliocene at Dry Creek bore. 
MyYocHAMA PLANA, spec. nov, Pl. xii, fig. 4. 
Left valve almost flat ; triangular, the sides diverging at about 
100°, posterior side the longer, anterior margin slightly arched, 
Surface with irregular, narrow, flat, concentric plications and faint 
radial markings, except in the umbonal region, which is lightly 
corrugated. Right valve flat, attached by the whole surface or 
partially only. Dimensions:—Antero-posterior diameter 38 ; 
anterior side 22, posterior side 22, umbo-ventral diameter 30 mm. 
Localities:—Miocene : Gippsland Lakes (five examples). Older 
Pliocene: Dry Creek bore (one attached valve). 
MyYocHAMA RUGATA, spec. nov., Pl. xii., fig. 5. 
Right valve oval-triangular, flat, wholly or partially attached. 
An example (from Gellibrand River), growing on a polyzoon by a 
