SILURIAN BIVALVED MOLLUSCA OF VICTORIA. 
The life provinces of the various horizons of the Victorian 
Silurian may then be studied with advantage. As, for instance, 
that of the sandstone of Moonee Ponds Creek, with their pre- 
vailing types of bracliiopods and ophiuroids ; and the shales 
and mudstones of South Yarra, with their more abundant 
bivalve and trilobite faunas. These two areas are apparently 
on the same stratigraphical horizon, but represent deposits laid 
down under different lithological conditions. 
At present the facilities for examining sections of strata in 
the various Silurian areas, by means of road and railway 
cuttings, and by borings, are not so good as in less recently 
developed countries such as England or the United States, and 
consequently this renders the work of correlation a difficult task. 
DESCRIPTION OF THE SPECIES. 
Class PELECYPODA. 
Family Solenopsidae. 
Genus Orthonota, Conrad, 1841. 
Orthonota australis, sp. nov. PI. I., Fig 1. 
Description. — Shell large, elongate; dorsal and ventral 
margins parallel; posterior margin well rounded. From the 
umbo to the posterior margin slightly more than twice the height 
of the shell. 
Valves rather strongly convex, moderately steep on the 
ventral margin, and sloping away towards the posterior cardinal 
area, where they are more compressed. 
Beaks situated close to the anterior extremity, rather 
prominent. 
Surface of the valves ornamented with a series of strong- 
concentric sulci, interrupted below the high umbonal ridge by the 
cincture, wffiich is limited on each side by a furrow. 
Measurements.— Approximate (from the specimen figured). 
Length, 42 mm. 
Greatest height, 18 mm. 
Affinities . — A British fossil (found also in Norway and 
Gotland), which shows some features in common with ours is 
Orthonota extrasulcata, Salter,* occurring in the Upper Ludlow 
* Mem. Geol. Surv. G. Brit., Vol. II., Pt. I., 1848, p. 361, PI. 17, Fig. 3. See alst 
Grammysia extrasulcata, Salter sp., McCoy, Brit. Pal. Fossils, 1852, p. 281, PI. IK. 
Fig. 29. 
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