[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 Great
     Britain,’ vol. iii. 1866.
(13) “On the Ancient Rocks of the St David’s Promontory, South Wales,
     and their Fossil Contents.”  Harkness and Hicks.—­’ Quart. 
     Journ.  Geol.  Soc.,’ xxvii. 384-402. 1871.
(14) “On the Tremadoc Rocks in the Neighbourhood of St David’s,
     South Wales, and their Fossil Contents.”  Hicks.—­’Quart. 
     Journ.  Geol.  Soc.,’ xxix. 39-52. 1873.