226 
SILUEIA. 
[Chap. X. 
transversalis, Dalm., Atrypa marginalis, Dalm., and A. reticularis, Linn., are far j 
more abundant in the Wenlock than in any of the lower strata, except the i 
Llandovery rocks. The last-named species ranges no lower than the Llan- J 
dovery strata ; and the same may be said of Spirifer exporrectus, Wahl. *, Sp. j 
plicatellus, Sow., PI. XXI. f. 2, Retzia cuneata, Dalm., and Rhynchonella bore- 
alis, Schl. ; and they are rare below the Wenlock shale. These species have a 
wide geographical range. In Chapters VI. and VII. thirty species are enume- > 
rated from the various strata of the Upper Silurian rocks, among which Lep- 
taena transversalis, Strophomena euglypha, Spirifer elevatus, Sp. plicatellus, and 
Pentamerus galeatus are most characteristic of the Wenlock strata j Rhyncho- 
nella Wilsoni, Rh. nucula, Rh. navicula, Orthis lunata, and Chonetes lata chiefly II 
distinguish the Ludlow rocks. Yet in some localities Rhynchonella navicula 
and Chonetes lata are prevalent in the Wenlock strata — the former near Builth, 
the latter on the west coast of Ireland. 
The species above enumerated are drawn in Plates XX.-XXII. There are, 
however, several others which must be noticed. Among the rarer Wenlock 
species may be reckoned Discina Morrisii of Davidson, D. Verneuilii, Dav., 
Orbiculoidea Forbesii (Foss. 58. f. 11), and Crania implicata, PI. XX. f. 4. 
Discina rugata, PL XX. f. 1, 2, and D. striata, f. 3, are common Ludlow 
forms. 
Siphonotreta Anglica, Foss. 58. f. 10, is a rare fossil. This genus is charac- 
teristic of the lowest Silurian rocks in Russia. The genus Obolus, also very 
characteristic of the same Lower Silurian near St. Petersburg, has, through the 
Fossils (58). Upper Silurian Brachiopoda. 
1. Ehynchonella nucula. 2. Eh. Lewisii, Davidson. 3. Atrypa Grrayii, Davidson. 
4. Eetzia Bouchardi, Dav. 5. E. Barrandii, Dav. 6. Porambonites Capewelli, Dav. 
7. Eetzia Salteri, Dav. 8, 9. Variety of the same (var. Baylei, Dav.). 10. Siphono- 
treta Anglica, Morris ; a, portion of the surface, with the annulated spines, magnified. 
11. Orbiculoidea Forbesii, Davidson. 
acumen of Mr. T. Davidson, been recognized in the Wenlock Shale of Dudley 
and Walsall. The British species, Obolus Davidsoni, and its var. transversus, 
Salter t, is a larger form than those of Russia. With such constantly augment- 
ing data for connecting them, it is obviously impossible to divide the Silurian 
rocks into two natural systems. 
Spirifer exporrectus, Wahl., is frequent in the Woolhope Limestone and 
Lower Wenlock Shales. Sp. sulcatus, Hisinger, is a small species common on 
* Wahlenberg gave the fossil this name before rian fossils in Mr. Davidson's Monograph (vol. i. 
Hisinger and Dalman termed it ' trapezoidalis.' Introd. p. 136), Palseontographical Society's Mo- 
| They are figured and described as Upper Sihi- nographs, 1854. 
