xviii DESCRIPTIVE REMARKS. 
Kington, Herefordshire, were probably of still greater dimensions. In 
the uppermost Ludlow rocks at Lesmahago, Lanarkshire, these re- 
markable Crustacea are very prevalent in the dark clay slate; P. 
bilobus, of which a reduced figure from a young specimen is shown, 
Pl., xxvii., fig. 5, is one of the species from this locality, by which it 
will be seen that the entire form of Pterygotus was very simple; a small 
semi-oval, or sub-quadrate head (or carapace), followed by twelve con- 
vex body-rings (segments), the last forming a pointed, truncate, or 
bilobed tail-joint (telson); two large compound eyes on each side 
of the carapace, beneath which the mouth is situated, protected by a 
large heart-shaped labrum (metastoma). Five pair of appendages are 
arranged round the mouth, and attached to the under surface of the 
carapace, including anteriorly a pair of large jointed antenne, with 
massive pincers (chele), furnished with sharp cutting teeth; next to 
these, one or two pairs of mandibles with serrate edges (endognaths), 
and posteriorly a pair of swimming feet (ectognaths).* 
The genus Pterygotus commences in Upper Llandovery strata, rang- 
ing through all the overlying Upper Silurian rocks into the Cornstones 
of the Old Red Sandstone. Nine species are included in the list of 
Fossils appended to Siluria, 4th ed., p. 521. Seven of these occur in 
Ludlow rocks, three of them continuing to the Passage beds. 
Eurypterus, though resembling Pterygotus, differs from it in several 
particulars, especially in the absence of the great pincer-like claws ; 
also in having the eyes within the cephalic shield, and not upon the 
border, as in Pterygotus. They have similar scale-like markings, 
though less conspicuous than in Pterygotus. Hight species have been 
found in Britain; seven of these occur in Ludlow rocks, two of them 
continuing into the Passage beds, and one other being peculiar to that 
uppermost division of the Ludlow rocks. 
Ei. pygmeus, Pl. xxvii., fig. 4, a smail and abundant species in the 
Tilestones at Kington and Ludlow, accompanies other species at the 
latter place; the genus continuing through the Old Red Sandstone 
into the Lower Beds of the Carboniferous rocks. Fragments of simi- 
lar large Crustacea have also been found in the Tilestones of West- 
moreland, and in these localities they are accompanied, as in Scotland, 
and at Ludlow, Kington, &c., by the small Lingula cornea, Pl. xxiv., 
fig. 6. 
Of the shrimp-like forms, Ceratvocaris papilio, Pl. xxvii, fig. 2, is 
a characteristic species pecuhar to the uppermost Ludlow rocks of 
Lesmahago, in Lanarkshire. This genus, which commenced in the 
Llandeilo rocks, with two species, is represented by eleven species in 
Lingula cornea, Pl, xxiv., fig. 6; Chonetes lata, fig. 11; Bellerophon expansus, PI. 
xxvi., fig. 5; Platyschisma helicites, fig. 3; Holopella obsoleta, fig. 2. 
* Although these remarkable Crustacea are related to the Pecilopoda to which the 
recent Limulus or King Crab belongs, Professor Huxley has shown their affinities to 
be closer to the Copepoda, and some points of structure to be comparable with the living 
forms of Calanus, Sappharina, &c. ‘Mem, Geol, Sury. Monograph 1, 
