THE CAMBRIAN PERIOD. 89 
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 1s worthy of 
note that the lowest of all the groups of 
the JAZollusca — namely, that of the Sea- 
mats, Sea-mosses, and Lace-corals (Po/y- 
zoa)—1is 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 Ht 
probability be made to this statement in Hint so Fsementiak 
favour of the singular genus Dayctyonema  Dictyonema soctale, con- 
; : : ote siderably enlarged, show- 
(fig. 33), which is highly characteristic of ing the horny branches, 
the jhighest Cambrian, beds (Tremadoc .. ti ten, comers 
Slates). This curious fossil occurs in the of cells on each side. 
form of fan-like or funnel-shaped expan- (0™8'*") 
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. Dzctyonema has generally 
been referred to the Graf/olites ; but it has a much greater 
affinity with the plant-like Sea-firs (Sertudarians) 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 palzontolo- 
gical relations of the Cambrian Rocks :— 
(1) ‘Siluria.” Sir Roderick Murchison. 5th ed., pp. 21-46. 
(2) ‘Synopsis of the Classification of the British Palzeozoic Rocks.’ 
Sedgwick. Introduction to the 3d Fasciculus of the ‘ Descrip- 
tions of British Palzeozoic 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 Cainbridge.’ 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 Bohéme.’ Barrande. Vol. I. 
(7) ‘ Report of Progress of the Geological Survey of Canada, from its 
Commencement to 1863,’ pp. 87-109. 
x 
