a large and varied development in the Silurian
deposits.

[Illustration: Fig. 33.--Fragment of _Dictyonema sociale_,
considerably enlarged, showing the horny branches, with their
connecting cross-bars, and with a row of cells on each side.
(Original.)]

An exception, however, may with much probability be made to this
statement in favour of the singular genus _Dictyonema_ (fig.
33), which is highly characteristic of the highest Cambrian beds
(Tremadoc Slates). This curious fossil occurs in the form of
fan-like or funnel-shaped expansions, composed of slightly-diverging
horny branches, which are united in a net-like manner by numerous
delicate cross-bars, and exhibit a row of little cups or cells,
in which the animals were contained, on each side. _Dictyonema_
has generally been referred to the _Graptolites_; but it has a
much greater affinity with the plant-like Sea-firs (_Sertularians_)
or the Sea-mosses (_Polyzoa_), and the balance of evidence is
perhaps in favour of placing it with the latter.

LITERATURE.

The following are the more important and accessible works and
memoirs which may be consulted in studying the stratigraphical
and palaeontological relations of the Cambrian Rocks:--

(1) 'Siluria.' Sir Roderick Murchison. 5th ed., pp. 21-46.
(2) 'Synopsis of the Classification of the British Palaeozoic Rocks.'
Sedgwick. Introduction to the 3d Fasciculus of the 'Descriptions
of British Palaeozoic Fossils in the Woodwardian Museum,'
by F. M'Coy, pp. i-xcviii, 1855.
(3) 'Catalogue of the Cambrian and Silurian Fossils in the Geological
Museum of the University of Cambridge.' Salter. With a Preface
by Prof. Sedgwick. 1873.
(4) 'Thesaurus Siluricus.' Bigsby. 1868.
(5) "History of the Names Cambrian and Silurian." Sterry
Hunt.--'Geological Magazine.' 1873.
(6) 'Systeme Silurien du Centre de la Boheme.' Barrande. Vol. I.
(7) 'Report of Progress of the Geological Survey of Canada, from its
Commencement to 1863,' pp. 87-109.
(8) 'Acadian Geology.' Dawson. Pp. 641-657.
(9) "Guide to the Geology of New York," Lincklaen; and "Contributions
to the Palaeontology of New York," James Hall.--'Fourteenth
Report on the State Cabinet.' 1861.
(10) 'Palaeozoic Fossils of Canada.' Billings. 1865.
(11) 'Manual of Geology.' Dana. Pp. 166-182. 2d ed. 1875.
(12) "Geology of North Wales," Ramsay; with Appendix on the
Fossils, Salter.--'Memoirs of the Geological Survey of Gr