Fossil Faiitia, Table Cape Beds, Tasmania. 137 



Dimensions. — Average dimensions of the Table Cape specimens : 

 Antero-posterior diameter, 29 mm. ; urabo-ventral diameter, 27 

 mm. ; thickness through both valves, 15 mm. 



Average diniensions of Spring Creek specimens : — Antero-pos- 

 terior diameter, 50 mm. ; umbo-ventral diameter, 45 mm. ; thick- 

 ness through single valve, 12-5 mm. 



The largest specimen yet to hand is from the Spring Creek 

 beds, which gives the following measurements : — Antero-posterior 

 diameter, 62 mm. ; umbo-ventral diameter, 57 mm. ; thickness 

 through the single valve, 15 mm. 



Locality. — Eocene beds of Table Cape, Tasmania. Seven double 

 valves and a single valve. Common in the lower eocene sandy 

 beds of Spring Creek, near Geelong, and the lower beds of 

 Maude, Moorabool Valley ; also from the eocene limestone at 

 Waurn Ponds (JVIcCann's Quarry). 



Observations. — This species is obviously closely related to Do- 

 sinia Johnstoni, Tate, better proof of which we could not have 

 than the fact that Professor Tate himself has recorded this very 

 characteristic miocene species as occurring in the eocene beds of 

 Table Cape and Spring Creek. The eocene and miocene shells 

 seem to me however to be sufficiently distinct, after long and 

 minute study, to warrant the desci-iption and the application of a 

 new name to the eocene form. 



In the lirst place, an important difference between the herein- 

 described species and D. Johnstoni, Tate, and one which the most 

 casual observer can hardly fail to detect at first sight, is the very 

 much closer, finer, and even more regular concentric ridging. In 

 Professor Tate's description of D. Johnstoni he states that the 

 concentric ridges are "separated Ijy linear deep sulci (about 

 twenty in a breadth of ten millimetres measured from the 

 ventral margin)." As the Table Cape shells are not very far 

 removed in dimensions from those given by Professor Tate for 

 D. Johnstoni, they will serve as a reliable basis upon which the 

 contrast of the concentric ornamentation may be indicated. 

 These Tasmanian examples give an average of forty-nine grooves 

 in the 10 mm. from the ventral margin as against the above. In 

 the examination of the Table Cape examples a noticeable feature 

 is that as the specimens increase in dimensions the concentric 

 ridges tend to become slightly less in number. This latter feature 



