7 
been exjircssed to tlic contrary, I cannot see any ditrcrcnce between the former 
and the illustration J)r. Cox lias kindly put at onr service. The natural size 
is identical in l)oth cases, the concentric costa the same, and method of 
distortion ([uite similar. The condition most frequently met with is that 
represented in Figure 1-, with the umbo projecting above the hinge border. 
I must confess to never having seen the characteristic Estheria surface- 
reticulation on any of the Dent’s Creek specimens, but that of those from 
IMoore Park is ligured (Fig. 5) from Dr. Cox’s drawing. This docs not 
convey a perfectly accurate idea of the ornament of the valves in Estheria, 
but is nevertheless probably correct. When freshly liroken core-surfaces are 
examined, each valve is seen to be covered Avith a thin pellicle of mineralized 
matter;' but neither this nor the cast below shoAV any trace of the pitted 
a])})earance to which one is accustomed in Estheria. On the other hand, 
some examples present a slight, although undoubtedly wrinkled appearance, 
which may ])crhaps agree Avith the Avrinkled surface shoAvn in some of Professor 
11. Jones’ figures of Estheria elliptica.^ On the other hand, the possible 
relation of these little fossils to those small and hardly studied bivalves 
scattered through the Coal Measures of Great Britain, and more particularly 
the Calcifcrous Sandstone Series of Scotland, generally referred to Anthra- 
comya, should not be oA'erlooked. On the Avhole, hoAvever, bearing in mind 
the structure illustrated in Fig. 5, it is more probable that the present fossils 
arc related to Estheria. 
The size of the valves from the Moore Park and upper horizons of the 
Dent’s Creek borings is remarkably uniform, varying from to 2 millemctres 
iiA the longest diameter of the vaU'es. But those obtained from the 1,915- 
foot, 1,932-foot, and 2,000-foot beds in the last-named exploration arc 
decidedly larger, measuring from 2 millemctres to as much as 5 millemetrcs. 
This dilference in size produces a notcAvorthy alteration in the appearance of 
tliesc little fossils ; and it would not surprise me to find that Ave have here a 
second species of Estheria. The material, unfortunately, is too limited in 
quantity to permit of more than a passing notice, Avith a figure for future 
reference. 
The examples from the Narrabeen Bore arc but fragmentary, but I 
believe them to be of the type just referred to. If so, coming from a depth 
of 1,9G1 feet, they Avill tend to bear out the supposition that a second species 
exists. 
* Mou. Foss, Estherioi, Pal. Soc., 1802, t, 3, Figs, 21 and 22. 
