PERMO-CARBONIFEROUS PELECYPODA — ETHERIDGE. 
187 
particularly in the case of the last with its deeply emarginate 
posterior wing. 
Log. and Horizon . — Mouth of Crooked River, near Gerringong, 
Illawarra District — Upper Marine Series. 
Genus Mytilops, J. Hall. 
(1st Report State Geol. N. York, 1884, p. 15.)* 
Mytilops ? ravensfieldensis, sp. nov. 
(PI. xxxiii., figs. 6 - 7). 
Sp.Char . — Shell (left valve) narrow, somewhat elliptical, oblique, 
generally mytiliform, gibbous and transversely arched posterior to 
the umbone ; hinge line faintly arched in the cast, but less than 
the length of the shell ; no ligamental furrows, but beneath the 
umbo is a single oblique cardinal fold, posterior teeth or folds none; 
ventral margin very obliquely inclined; umbone terminal, no 
anterior end ; posterior end, or general body of the shell convex 
immediately behind the umbone, gradually flattening towards the 
rounded posterior margin, where the valve is broad ; anterior 
muscular impression, invisible and the posterior very faint and 
high ; pallial impression well marked and continuous, the exterio- 
pallial margin wide; sculpture unknown, but probably concentric 
and non-radiate. 
Obs . — Allied either to Mytilops or Mytilarca . It possesses 
the outline of the latter genus, and approaches the former in its 
simpler hinge structure ; differs from Mytilarca in the absence 
of a ligamental area and posterior teeth, and from Mytilops in 
the presence of the cardinal fold. So far as our Permo-Carboni- 
ferous fauna is concerned this is again an undescribed form. 
Log. and Horizon. — Ravensfield Quarry, near Farley — Lower 
Marine Series. 
# See note to preceding page. 
