THE CAMBRIAN PERIOD. 



8 9 



we have but few traces, and these wholly confined to the higher 



beds of the formation. We meet, however, with examples of 



the wonderful genus Orthoceras, with its 



straight, partitioned shell, which we shall 



find in an immense variety of forms in the 



Silurian rocks. Lastly, it is worthy of 



note that the lowest of all the groups of 



the Mollusca namely, that of the Sea- 



mats, Sea-mosses, and Lace-corals (Po!y- 



zod] is only doubtfully known to have 



any representatives in the Cambrian, 



though undergoing a large and varied 



development in the Silurian deposits. 



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 expan- 

 sions, 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 ani- 

 mals 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. 



Fig. 33. Fragment of 

 Dictyonema sociale, con- 

 siderably enlarged, show- 

 ing the horny branches, 

 with their connecting 

 cross-bars, and with a row 

 of cells on each side. 

 (Original.) 



'LITERATURE. 



The following are the more important and accessible works and memoirs 

 which may be consulted in studying the stratigraphical and paloeontolo- 

 gical 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 'Descrip- 

 tions 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 Pref- 

 ace 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. 



