Cyclonema, Euomphalus, Helicotoma, Holopxa, Holopella, Murchisonia, 
Ophileta, Platyschisma, Pleurotomaria, Raphistoma, and Subulites. 
Some heteropod forms occur, e.g. Bellerophon and Maclurea ; but 
pteropods are more frequent, being represented sometimes abun- 
dantly by the genera Tentaculites (regarded by some as an annelide), 
Hyolithus (or Theca), Conularia, and Pterotheca. That the salt waters 
of the Silurian era swarmed with cephalopods may be inferred from 
the fact that according to Barrande’s census no fewer than 1622 
species have been described. They are all tetrabranchiate. Some 
of the most abundant forms are straight shells, of which Orthoceras 
(Figs. 324, 328) isthe type. ‘This characteristically Paleozoic genus 
abounded in the Silurian period and many of its individuals attained 
a great size. Barrande has described upwards of 550 species from the 
basin of Bohemia. Of Cyrtoceras, in which the shell was curved, the 
same small area has yielded more than 330 species. Phragmoceras 
(Fig. 328) likewise possessed a curved shell, but with an aperture con- 
tracted in the middle. In Ascocevas the shell was globular or flask- 
shaped, with curiously curved septa; in Ltuites (Vig. 328) it was 
curled like that of Nautilus. The two latter genera occur in Silurian 
rocks, but while Litwites never outlived the Silurian period, Nautilus 
is still a living denizen of the sea. | 
The first traces of vertebrate life make their appearance near the 
top of the Silurian system. ‘They consist of the remains of fishes, the 
most determinable of which are the plates of placoderms (Pteraspis, 
Coccosteus). ‘The bone-bed of the Ludlow rocks has also yielded 
certain curved spines which, under the name of Onchus, have been 
referred to a cestraciont, and some shagreen-like plates which have 
been supposed to be scales of placoid fishes (Sphagodus, Thelodus), and 
‘ bodies like jaws with teeth which have been regarded as jaws of 
fishes (Plectrodus). It is possible, however, that some at least of 
these remains have been incorrectly determined, and may be 
crustacean. The Upper Silurian rocks have yielded, both in Europe 
and North America, great numbers of minute tooth-like bodies which 
were named “Conodonts” by their discoverer, Pander, and were 
supposed to be the teeth of such fishes as the lamprey, which 
possessed no other hard parts for preservation. ‘These bodies have . 
been also referred to different divisions of the invertebrata, their true 
position being still matter of dispute. 
§ 2. Local Development. 
Britain.'—In the typical area where Murchison’s discoveries were 
first made he found the Silurian rocks divisible into two great and well- 
marked series, which he termed Lower and Upper. This classification 
1 See Murchison’s “Silurian System,” and “Siluria;’’ Sedgwick’s “ Synopsis” 
(cited p. 652); Ramsay’s “ North Wales” in Memotrs of Geol. Surv. vol. iii,; Etheridge, 
Address Q. J. Geol. Soc. 1881; numerous local memoirs in recent volumes of the Quart. 
Journ, Geol. Soc, and Geol. Mag., particularly by Hicks, Ward, Hughes, Keeping, Lap- 
worth, &e., 

666 STRATIGRAPHICAL GEOLOGY.  [Boox VL 
