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same depth ; their cardinal extremities are rounded or angular, and in the 
latter case a little depressed ; their beaks are so strongly recurved as nearly 
to meet each other ; the hinge-area is moderately raised, its surface is curved, 
and it has a broad deltoid fissure partly covered by a pseudodeltidinm. The 
ventral furrow is shallow, and united to the sides by a regular curve of the 
shell; it is ornamented with from four to eight rather thin, longitudinal 
ribs, rarely bifurcating. The dorsal ridge is more distinctly limited than the 
ventral furrow, and its surface is regularly rounded, except for the ribs with 
which it is ornamented. Both valves are covered with a variable number of 
radiating, generally rounded, rarely angular, lateral folds. In young specimens 
most of these folds are simple, while in adults the two or three folds adjoining 
the ventral furrow and the dorsal ridge are Ijifurcated. In well-preserved 
specimens the surface ornamentation is seen to consist of fine lines of growth 
crossed at right angles by radiating striae, thus producing a fine reticulated 
design, invisible to the naked eye. The spiral supports are twisted so as to 
produce a rather ellijotic cone ; the muscular impressions are small, and offer 
no special point of interest. 
Dimensions. — The dimensions of this species being very variable, I 
will give those of the best Australian specimen. The length of this is thirty- 
six millimetres, the breadth fourteen, and the thickness about twenty. 
Delations and Differenees. — This species cannot easily be distinguished 
from its congeners owing to the extent of its variations. It needs some 
experience to avoid confounding it with S. mosqiiensis, Bischer, and S', siriatus, 
Martin, the folds of which are more regular and more numerous in the first, 
and much more divided in the second. 
Horizon and Loealities . — This species is very abundant in the upper 
beds of the Carboniferous Limestone at Vise, also in Yorkshire, Scotland, and 
Ireland. It has been found at Bleiburg in Carinthia. Mr. Clarke found it 
in Muree Quarry, at Branxton, St. Hcliers, Mulbring Creek, A^llalong,, 
Burragood, Colocolo, Cedar Brush, and in the neighbourhood of Tilleghary, 
and Jervis Bay. Bew Carboniferous fossils in Australia occupy so extensive 
a horizontal position as this one. Mr. Etheridge records with doubt its 
occurrence at Bowen Biver, Queensland. 
