DESCRIPTIVE REMARKS. xlvil 
in structure and contents of the three subdivisions of the highest Silu- 
rian formations, being also remarkable for exhibiting a transition into 
the next overlying series, the Old Red Sandstone or Devonian forma- 
tion. ‘The lowest stratum is a calcareous shelly band, which contains 
asmall Brachiopod shell, Rhynchonella navicula, (Sil., Pl. xxii., fig. 1, 2,) 
in profusion ; over this are grey argillaceous beds, which are sometimes 
spheroidal and concretionary, and from their incoherent character 
easily decompose into a kind of mud. The upper and principal por- 
tion is usually a thin bedded calcareous stone, slightly micaceous, and 
of a bluish-grey colour, and remarkable for its symmetrical transverse 
joints : these upper beds are full of the following, with other fossils: 
Discina rugata, Pl. xxiv., fig. 7 ; Rhynchonella nucula, fig. 9; ,Orthis 
lunata, fig. 10; Chonetes lata, fig. 11; Pterinea retroflexa, Pl. xxv., fig. 
4; Orthonota amygdalina, fig. 9; Goniophora cymbeformis, fig. 11; 
Cyclonema corallii, Pl. xxiv., fig. 1 ; the curved Annelid-tube, Serpu- 
lites longissimus, Pl. xxvi., fig. 10; Cornulites serpularius, P1., xix., fig. 
2, and the minute Beyrichia Kladeni, Pl. xxiii, fig. 9. Very few 
corals occur except the small incrusting species, Alveolites fibrosus, 
which is frequently found covering small univalve shells, such as Cy- 
clonema corallit, Pl. xxiv., fig. 1; and Murchisonia corallit (Sil. Pl., xxiv., 
fig. 7). Orthocerata (occasionally of large size) of similar species to 
those of the Lower Ludlow rock are not unfrequent. Of Trilobites 
Phacops caudatus, Pl. xxiii., fig. 5; Knerinurus punctatus, fig. 2, and 
Calymene Blumenbachit, fig. 1, pass through the series, but are not 
abundant: the most prevalent Trilobite is Homalonotus Knightii, PI. 
XXvil., fig. 1, found throughout the whole range of this formation from 
Pembrokeshire to Westmoreland, and sometimes of large size. The 
highest member of the Ludlow rocks is most interesting, as until re- 
cently it was described by Sir R. Murchison as being the oldest rock 
in which fossil fish had been found; the only exception being that 
already alluded to—the discovery of a fragment of Pterasp7s in the cen- 
tral part of the same formation. The uppermost Ludlow rock also con- 
tains the earliest remains of land plants (‘ Siluria,” p. 133). 
The largest, if not the most highly organized Crustacean in Silurian 
strata is the Pterygotus problematicus, Pl. xxvii., fig. 3; this charac- 
teristic species of the order Eurypterida* has a greater stratigraphical 
range than any of the others, commencing in Wenlock strata, and con- 
tinuing to the Passage beds or Uppermost Ludlow. The size of this 
species, from a comparison of its parts discovered at Ludlow, with that 
of others found nearly entire in Scotland is estimated as being not much 
less than five or six feet in length, whilst P. Zudensis, also found near 
Ludlow, in the Passage beds, and P. gigas, from the Tilestonest of 
* Professor Huxley includes Pterygotus and Eurypterus under this order. 
+ The Tilestones (formerly included with the Old Red Sandstone, but now classified 
by Sir R. Murchison, ,with the Silurian rocks, as containing species common to the 
Upper Ludlow formation), in their range from Shropshire, through Herefordshire, Rad- 
nor, Brecon, and Caermarthen, are characterized by the following, amongst other fossis: 
