162 
Strophomena rhomhoidalis, var. analoga, T. Davidson, I860, Mon. Garb. Brach. 
Scotland, p. 31, pi. 1, fig. 26-33 (non Wilckens). 
,, ,, var. analoga. Idem, 1861, Geologist, lY, p. 47 (non 
Wilckens). 
,, ,, var. analoga, Idem, 1861, Mon. Brit. Garb. Brach., p. 
119, pi. 28, fig. 1, 2 (non Wilckens). 
,, analoga, J. W. Dawson, 1868, Acad. GeoL, p. 296, fig. 95. 
,, rlioniboidalis, var. analoga, E. Etheridge, 1872, Quart. Journ. Geol. Soc., 
XXVIII, pp. 331 and 333, pi. 15, fig. 3, and pi. 16, fig, 7. 
,, depressa, E. Toula, 1874, Sitzung. K. Akad. Wissen. Wien, LXXI, 
p. 548, pi. 2, fig. 8 (non Sow.). 
This shell is usually sub-trapezoidal, sometimes a little irregular, 
broader than long ; the valves are geniculated ; the hinge line is straight, 
oecujiying the greatest breadth of the shell ; its ears are generally depresseti, 
with rounded extremities, extending beyond the lateral margins to which 
they are joined by a regular curve. The ventral valve is slightly ventricose 
near the beak, while the rest of its reticulated surface is nearly flat ; its 
surface is ornamented with concentric folds, varying in number and extent, 
according to the age of the individual. Starting from the last of these folds, 
the shell recurves on itself at nearly a right angle, prolonging itself for a 
greater or less distance in this new direction. The concentric folds over the 
visceral region are not always regular, hut frequently interrupted or wavy. 
In adult specimens the last formed concentric folds are curved outwards as 
they approach the hinge line, and form with it an acute angle ; while the 
first formed {i.e., those in the centre of the valve) generally meet the hinge 
line at a right angle. The whole external surface is ornamented with fine 
radiating ribs, crossing the concentric folds at right angles. The beak is 
small, not recurved, often perforated, in young individuals, by a circular 
opening, which completely disappears in the adult. The dorsal valve is 
slightly concave in the visceral region, where it is some distance from the 
ventral valve, while the geniculated part approaches it, and follows all its 
cmwes. The external ornamentation is like that of the ventral valve. I 
think I may dispense with a description of the internal structure of the valve, 
because it has been so perfectly described and figured by Mr. Davidson, and 
because PI. IX, Pigs. 3a and 36, represent that of the Australian specimens. 
Dimensions . — The dimensions of this species are very variable. The 
largest specimen sent by Mr. W. B. Clarke is five centimetres broad by three 
long. 
