HAMILTON GROUP. 
203 
4. Atrypa -. — Obtusely cuneate ; valves equally convex, with about 16 plicae on each, 
three of which are raised in front; beak of the lower valve prominent, small, that of the upper 
closely pressed to the lower. 
This shell often varies in size from the figure. 
Locality —Eighteen-mile creek. 
5. Atrypa -, n. s. — Broadly obovate, tapering abruptly to the beak, and rounded be¬ 
low ; lower valve most convex; beak of the lower valve large, very slightly incurved ; that of 
the upper valve small, rather prominent, and closely pressed to the lower valve. 
This shell is distinguished from figs. 2 and 3, by its broadly rounded base, the large, pro¬ 
minent, nearly straight beak of the lower valve, and the slight radiating elevated lines which 
are visible in the cast. 
Locality —Eighteen-mile creek. 
6. Delthyris sculptilis, n. s. — Semicircular, somewhat auriculate ; surface marked with 
eight prominent ribs, which are crossed by strong elevated laminae of growth, the spaces be¬ 
tween which appear as if grooved; the mesial elevation of the upper valve scarcely larger 
than the ribs. 
At first view this fossil appears much like the D. zigzag, but there are fewer ribs, and the 
laminae are more deeply sculptured. So far as known, this fossil is confined to the Encrinal 
limestone. 
Locality —Eighteen-mile creek. 
The following species of Avicula is associated with the Encrinal limestone of this group 
throughout the district. 
82. 
1 and 2. Avicula decussata. 
1 and 2. Avicula decussata, n. s.— Obliquely ovate or sub-rhomboidal; surface marked 
by numerous fine radiating lines, which are decussated by irregular concentric lines of growth : 
the concentric lines are rather like wrinkles, and stronger than the radiating lines. 
26 * 
