682 -  STRATIGRAPHICAL GEOLOGY.  [Boox VL 
bed,” is the oldest from which any considerable number of vertebrate — 
remains has been obtained. In spite of its insignificant thickness it has 
been detected at numerous localities from Ludlow as far as Pyrton. 
Passage, at the mouth of the Severn—a distance of 45 miles from north 
to south, and from Kington to Ledbury and Malvern—a distance of 
nearly 30 miles from west to east; so that it probably covers an area 
(now largely buried under Old Red Sandstone) not less than 1000 square 
milesin extent. Yet it appears never to exceed and usually to fall short 
of a thickness of 1 foot. Fish remains, however, are not confined to this 
horizon, but have been detected in strata above the original bone-bed at 
Ludlow. The higher parts of the Ludlow rock consist of fine yellow 
sandstone and harder grits known as the Downton sandstone. Originally 
the whole of these flaggy upper parts of the Ludlow group were called 
“'Tilestones ” by Murchison, and being often red in colour were included 
by him as the base of the Old Red Sandstone, into which they gradually 
and conformably ascend. Undoubtedly they show the gradual change of 
physical conditions which took place at the close of the Silurian period 
in the west of England, and brought in the deposits of the Old Red 
Sandstone. But as their organic contents are still unequivocally those 
of the Ludlow group, they are now classed as the uppermost zone of the 
Silurian system. 
A considerable suite of organic remains has been obtained from the 
Upper Ludlow rock, which on the whole are the same as those in the © 
- zones underneath. Vegetable remains, some of which seem to be fucoids, 
but most of which are probably terrestrial and lycopodiaceous, abound 
in the Downton sandstone and passage-beds into the Old Red Sandstone. 
Some minute globular bodies, doubtfully referred to the sporangia of 
a lycopod (Puchytheca), occur with some other plant remains (Pachy- 
sporangium, Actinophyllum, Chondrites, a beautiful sea-weed). Corals, 
as might be supposed from the muddy character of the deposit, 
seldom occur, though Murchison mentions that the encrusting form 
Alveolites fibrosus may not infrequently be found enveloping shells, 
Cyclonema corallii and Murchisonia corallii being, as their names imply, its 
favourite habitats. All the corals of these and the other divisions of 
the Ludlow group are also Wenlock species. Some annelides (Serpulites 
longispinus, Cornulites serpularius, and Trachyderma coriacea) are not un- 
common. ‘The crustacea are represented in the Upper Ludlow rock 
by 23 genera and 71 species, and in the whole Ludlow group by 29 
genera and 97 species, including ostracods (Beyrichia Kledeni, Leperditia 
marginata, Entomis tuberosa), phyllopods (16 species, Ceratiocaris, Dictyo- 
caris), and eurypterids (Hurypterus 10 species, Hemiaspis 6, Pterygotus 9, 
Slimonia 3, Stylonurus 3, Himantopterus 1). The trilobites have still 
further waned in the Upper Ludlow rock,{though Homalonotus Knightit, 
Encrinurus punctatus, Phacops Downingize, and a few others still occur, and 
even the persistent Oalymene Blumenbachii may occasionally be found. 
Of the brachiopods the most abundant forms in this zone are Rhyn- 
chonella nucula, Chonetes striatella, Discina rugata, and Lingula cornea. 
The most characteristic lamellibranchs are Orthonota amygdalina, Gonio- 
phora cymbeformis, Pterinea lineata, P. retroflexa ; some of the commonest 
gasteropods are Murchisonia corallii, Platyschisma helicites, and Holopella 
obsoleta. The orthoceratites are specifically identical with those of the 
Lower Ludlow rock, and are sometimes of large size, Orthoceras bullatum 

