MIDDLE BALA GROUP. 
57 

Case and 
Column of 
Drawers. 
i] 
Reference to McCoy's 
Synopsis: and Figures of Genera, 
Names and References; Observations, &c. 

Gl 
Gl 
Gl 
Gl 
Gl 



p- 213. 
O. callactis, p. 214. 
p. 213. 
p. 214. 
O. plicata, p. 214. 
Orthis, Dalman. A genus of all others charac- 
teristic of the great Bala and Silurian groups. 
Less abundant im Devonian, and not quite 
extinct in Carboniferous times. It is easily 
distinguished by its convex form, and single 
cardinal process, from Leptena or Stro- 
phomena, which genera occur with it, and 
of large size. 
Orthis Actoniz, Sowerby (Siluria, 2nd ed. pl. 
5, fig. 11), and Salter, Mem. Geol. Surv. m1. 
p. 339, pl. 21, figs. 1—8. A shell easily 
distinguished from its associate O. flabel- 
lulum, by haying the opposite valve convex; 
the concave one being the dorsal one. 

Orthis, var. (as O. callactis, but not of Dalman). 
It is only a large variety of O. Actonie. 
Orthis biloba, Linn. (Siluria, 2nd ed. pl. 20, 
| fig. 14). Not uncommon in Bala rocks, 
Swarms in Wenlock. 
Orthis calligramma, Dalman (Salter, in Mem. 
Geol. Surv. U1. p. 335, pl. 22, fig. 1). Both 
valves are equally convex in all the varieties. 

| Orthis, id. var. plicata, Salter (0. plicata, Silu- 
rian System, pl. 21, fig. 6). O. calligramma, 
var. plicata, Salter, Mem, Geol. Surv. U1. 
p. 336, pl. 22, fig. 5). 


Numbers and Localities. 
Bryn Eithin, Penmachno. 
Gelligrin; Bryn Melyn, 
&e., near Bala; Cader 
Dinmael; Pont - y - glyn, 
Diffwys; Blaen - y - ewm, 
Nantyr; Llansantfraid, Glyn 
Ceiriog; Alt - yr- Anker, 
Meifod; Tyn-y- Cabled, 
Llanfyllin. a. 149, Acton 
Scott, Shropshire;  In- 
gleton; Thornton; Horton; 
High Haume. 
Gaerfawr, Welchpool; Bala. 
One very large variety at 
Wrae Quarry, Up. Tweed. 
Cefn Goch, Glyn Ceiriog. 
Llansantfraid, Glyn Ceiriog. 
[The typical form of the 
species is of Lower Bala 
age in Britain. ] 
Gelligrin; b. 210, Bryn Me- 
lyn, Bala; Cwm of the Cym- 
merig, Bala;  Llansant- 
fraid, Glyn Ceiriog; Bryn 
Evan, Penmachno; Pwll- 
heli; Cader Dinmael, near 
Meifod, abundant. Gaer- 
fawr, Welchpool; Das Ei- 
thin; Horton in Ribbles- 
dale, 
