Chap. IX.] LOWER SILURIAN BRACHIOPODS. 
193 
0. calligramma, Dalm., and particularly O. striatula, Emm., just mentioned, are 
the characteristic Llandeilo species ; while 0. vespertilio, Sow., and 0. flabellu- 
lum, Sow., are very common in the Caradoc Sandstone, There is not much differ- 
ence in the distribution of these common Brachiopods in the two groups of 
rocks, except as regards their proportionate numbers in each ; but most of the 
other species above mentioned do not range lower than the Caradoc formation, 
while 0. elegantula and 0. calligramma occur in the Upper Silurian, — the former 
abundantly. 
Fossils (36). Lower Silurian Brachiopods. 
2 
1. Lingula attenu- 
ata, Sow. 2. Stropho- 
mena expansa, Sow. ; 
upper or dorsal valve. 
3. Orthis striatula, 
Emmons. 4. Orthis 
biforata, Schlotheim. 
5. O. porcata, M'Coy. 
6. Leptseiia sericea, 
Sow. 
Five at least of the above species, O. insularis,Eichw., 0. testudinaria, Dalm., 
0. elegantula, Dalm., O. calligramma, Dalm., and O. biforata, Schlot, Foss. 36. 
f. 4, are common in the Lower Silurian of North America, Russia, and 
Sweden; and are everywhere abundant in those British localities originally 
defined by me as Lower Silurian, as well as in North Wales and Ireland. The 
last-named two species occur, though rarely, in the Wenlock limestone of Britain. 
The same is the case with 0. porcata, M'Coy, Foss. 36. f. 5, which has also been 
found in that limestone at Dudley. In the Western States of America this shell 
is known under various names. It is the 0. occidentalis of Hall's work, but had 
been previously described, though from indifferent specimens, by M'Coy. This 
shell is one of our most characteristic Lower Silurian (Caradoc) types. Orthis 
biloba of Linne,Pl. XX. f. 14, which is abundant in the Wenlock Shale, has been 
detected in the Lower Silurian rocks of South Wales, and at the Chair of Kil- 
dare, Ireland. 
The closely allied genus Strophomena, consisting chiefly of the flatter forms 
of the so-called ' Orthis ' of the ' Silurian System,' exhibits some very marked 
and frequent species which are eminently typical of the Lower Silurian. These 
are : — Strophomena expansa, PI. VI. f. 4, and Foss. 36. f. 2 ; S. tenuistriata, p. 68, 
Foss. 13. f. 8 ; and S. grandis, PI. VI. f. 6, 7, which abound in the Lower Si- 
lurian rocks, and have been formerly given in the ' Sil. Syst.' as species of 
Orthis and Leptaena. They not only prevail in the Llandeilo and Bala deposits 
of Wales, but are common in the strata of the same age in Westmoreland, and 
in the Caradoc Sandstone of Shropshire. 
Strophomena corrugata, Portlock, is a highly ornamental species of the genus, 
with radiating lines and transverse undulations ; it is found in Wales and Ire- 
land. Another shell, probably of this genus, and frequent at Bala, is the common 
Caradoc species, Orthis spiriferoides, M'Coy, Foss. 37. f. 2, which is so like 
Spirifer plicatellus, Linn. (var. radiatus, Sow.), of the Upper Silurian rocks, as to 
be easily mistaken for it. 
The universally spread Leptsena depressa (Sil. Syst., now more correctly re- 
o 
