DESCRIPTIVE REMARKS. xlix 
the Ludlow rocks of Ludlow, Kendal, and the south of Scotland. 
Hymenocaris vermicauda, Pl. iv., fig. 1, characteristic of the Lingula 
Flags (Primordial Silurian), as already noticed (ante, p. xvi.), being 
the earliest representative of the order of bivalve Crustaceans, the 
Phyllopoda, to which Ceratiocaris is also referred. 
Fis Remarys are most frequent at the uppermost portion of the 
Ludlow formation, in what is called the Bone-bed, although, as before 
remarked, the first indication of this class of the Vertebrata has been 
met with in the lower part of the same Ludlow formation, in which was 
found one of those remarkable heads called Pteraspis—a ganoid fish 
allied to Cephalaspis, of which there are two species in the Ludlow 
rocks—P. truncatus, Pl. xxvii., fig. 6, and P. Banksit—both from the 
Passage beds of Kington, in Herefordshire, as well as from the Bone 
bed of Ludlow. In the same bed other fish remains have been found; 
Onchus tenuistriatus, Pl. xxvii., fig. 7., and O. Murchisoni, fig. 8, bony 
fin-spines resembling those of placoid fishes of the present day. The 
very small cushion-like bodies called Thelodus parvidens, fig. 11, which 
occur so abundantly in the stratum as often to constitute large por- 
tions of its thin layers, are believed to be the granules of the skin or 
shagreen of one or other of these two common species. The remark- 
able jaws and teeth, Plectrodus mirabilis, fig. 9, and P. pustulvferus 
(figured in ‘ Siluria’’), probably belonged to some small ganoid fish, 
Fish and Crustacean remains have also been found near Ludlow in 
strata, proved to be at a higher level than the original Bone-bed ; the 
fish remains consisting of fragments of Plectrodus, fig. 10; the Onchus 
Murchisoni, fig. 8, with two species of Auchenaspis, buckler-headed fish, 
also allied to Cephalaspis. ‘‘It may therefore,” as the author of 
‘« Siluria” remarks, ‘‘ be naturally suggested that this band constitutes 
the last link in the chain of Silurian life.” 
