144 
SILURIAN. 

| 
Case and | 
Column of 
Drawers. 

Gc8 
Gc8 
FC 
Gd2 
FC 

Reference to McCoy’s 
Synopsis: and Figures of Genera. 

do. 


Names and References; Observations, &c. 

Orthis, Dalm. Of all Brachiopod genera the 
most characteristic of Lower Palaeozoic 
rocks. Only a few forms, and those of a 
particular group (0. filiaria, O. Michelinz) 
range into Carboniferous rocks. A wide 
hinge line, tolerably flat shells (sometimes 
bilobed), triangular foramen (open), and ab- 
solute want of spines on the surface, easily dis- 
tinguish it from Producta and its allies, and 
the single (not double) cardinal process (i.e. 
pedal muscle support) from Strophomena. 
It is hardly ever bent as in the last-named 
genus, and the valves are almost equally 
convex in many cases; in others the dorsal, 
rarely the ventral, is the flat one. Only one, 
O. calligramma, of the coarse-ribbed forms, 
comes up from Cambrian or Cambro-Silurian 
rocks to mix with the striated and faintly- 
ribbed forms characteristic of the Silurian. 
Orthis calligramma, Dalman. (De Vern. Geol. 
Russia, t. 13, fig. '7; Salter, in Mem. Geol. 
Sury. Vol. 1. Pt. 1, p. 374, for description. 
Also Vol. 111. pl. 22, and p. 335 to p. 387, 
Davidson, Sil. Brach. p. 240, pl. 35, figs. 1—17). 
Orthis, var. Davidsoni, De Vern. (Bull. Soc. 
Géol. de France, Vol. v. 2nd ser. pl. 4, fig. 9). 
Orthis rustica, Sow. (Sil. 2nd ed. pl. 20, fig. 10. 
Davidson, Lond. Geol. Journ. p. 64, pl. 13, 
figs. 1—4). 
Orthis rustica, var. rigida, Davidson (Lond. 
Geol. Journ. p. 63, pl. 13, figs. 16, 17), evi- 
dently an irregularly grown variety of the last. 
Orthis, var. Walsalliensis, Davidson (Bull. Soc. 
Géol. de France, 2nd ser. Vol. v. pl. 4, fig. 7). 
I think only a many-ribbed variety of O. 
rustica. 
Orthis biloba, Linn. (Ter. sinuata, J. Sowerby. 
Delthyris cardiospermiformis, von Buch). 
One of the prettiest fossils of the limestone, 
and most instructive as shewing (with Tere- 
bratula diphya and others) the double-valved 
nature of the dorsal valve in Brachiopods. 
(Siluria, 2nd ed. pl. 20, fig. 14. Davidson, 
Brit. Fos. Brach. Introd. pl. 8, fig. 141.) 


Numbers and Localities. 
b. 766, Dudley, F.C. 
b. 767, Dudley, F.C. 
a. 741, dorsal valve; a. 742, 
ventral valve, Dudley, F.C. 
b. 768, Dudley. 
Dudley. 
a. 755, dorsal valve; a. 756, 
ventral, Dudley, F.C. 
