192 SILURIA. [Chap. IX. 
deilo formation, occurring plentifully at Builth and Llandeilo in South Wales, 
and at Shelve in Shropshire. Other species will be mentioned afterwards. 
But by far the most abundant Bivalve Shells of these rocks belong to the 
group of Brachiopods represented more particularly by the genera Orthis, Lep- 
taena, and Strophomena. With these are associated, but much more rarely, 
some forms of Rhynchonella*. The genus Atrypa, as well as Pentamerus, has 
often been quoted from the Lower Silurian ; but, as will presently be shown, the 
various species of these last two genera are almost exclusively found in the 
middle and upper portions of the system. 
Although all the well-established forms known at the period of the publica- 
tion of the ' Silurian System ' are reproduced from the original etchings in 
Plates V. and VI., attention will here be specially directed only to those which 
are found in abundance, and are therefore useful indicators, either as charac- 
terizing one zone or band of strata, or, being found to ascend through a number 
of beds, showing that they lived through a long period of primeval time. 
The genus Orthis was one of the most prolific forms of the Brachiopoda in this 
period. The species have generally a roundish outline, with one valve convex, 
the other flat, and the hinge-line straight. Those which most characterize the 
Lower Silurian are arranged in the Plates above mentioned ; and some are 
figured in the two following woodcuts. 
The species which are abundant in the great mass of the Llandeilo Flags and 
Caradoc Sandstone are the following : — Orthis elegantula, PI. V. f. 5 ; O. testu- 
dinaria, PL V. f. 1, 2 ; 0. vespertilio, PI. VI. f. 1,2,8; and the simple-plaited spe- 
cies figured in the following woodcut, Foss. 35, viz. 0. flabellulum (f. 1), 0. Ac- 
toniae (f. 2), and the variable O. calligramma (f. 3, 4). 
Fossils (35). Lower Silurian Brachiopods. 
Some of the characteristic Lower 
Silurian forms of simple-plaited 
Orthides. 
1. Orthis flabellulum, Sow. 
2. O. Actoniae, Sow. 
3. 4. O. calligramma, Dalman : 
two varieties (3. virgata, Sil.Syst. ; 
4. plicata, Sil. Syst). 
Among other species frequent in the lower division, and, as far as I know, 
peculiar to it, are : — 0. insularis, Pander, not here figured f ; 0. alternata, Sow., 
PI. VI. f. 5; 0. confinis, Salter; O. striatula, Emmons, Foss. 36. f. 3, not pub- 
lished in the ' Silurian System,' but a very common British species, and the 
most abundant Orthis of this epoch in North America. 
0. porcata, O. turgida, O. sagittifera, and O. Hirnantensis, described by M'Coy, 
are other species of the Lower Silurian, found in Wales. 
Wherever several of these forms of Orthis are found, the geologist may be 
pretty sure that he is working in Lower Silurian rocks j and among these species, 
it should be noted that Orthis Actoniae, the more simple-ribbed varieties of 
* The relationships of all the Silurian so-called bonites intercedens, which is known under several 
Rhynchonella} are not yet determined. names in Dr. Pander's work, has been described 
t Of this common and characteristic species a by Professor M'Coy from the Lower Silurian beds 
good figure will be found in the G-eol. of Russia, of the Wrae Quarry, Peebles-shire, 
vol. n. pi. 8. f. 7. A much rarer fossil, the Poram- 
