_The Cambrian Faunas._ The Cambrian Period has been termed the age of trilobites, for they are the dominant forms of the time, but they are associated with many other forms of invertebrata; indeed all the great groups of this division are represented in the earliest Cambrian fauna. Dr C. D. Walcott records representatives of Spongiae, Hydrozoa, Echinodermata, Annelida, Brachiopoda, Lamellibranchiata, Gastropoda, Pteropoda, Crustacea and Trilobita as occurring in the _Olenellus_ beds of North America and other groups are represented in the rocks of this age in the Old World. The Cambrian trilobites as a whole are of more generalised types than those of the later systems which furnish their remains, as indicated especially by the looseness of the body, and the large number of body rings in many of the genera, while the tail or pygidium was small and formed of only a few coalesced segments, as pointed out by Barrande. In the later trilobites the test is more compact, there are on the whole fewer body rings, as more of these have become fused into a tail which is therefore larger than that of the average tail of the Cambrian trilobite. Taking the faunas in order, the oldest or _Olenellus_ fauna has furnished a great variety of forms in the North-west Highlands of Scotland, Shropshire, Scandinavia, Esthonia, Sardinia, Canada, and Newfoundland, whilst representative species of the fauna have been recorded also from Worcestershire, Warwickshire, Pembrokeshire, India, China, and Australia. The dominant form is the trilobite of the genus or group _Olenellus_, which contains a great variety of species referable to three or four divisions which have been ranked as separate genera by some writers. Associated with _Olenellus_ are trilobites belonging to other genera, which are found in higher deposits, though there represented by different species. Brachiopods are fairly abundant, especially those provided with a horny shell; of these, the genus _Kutorgina_ is widely distributed. The zoological relationships of several of the fossils of this horizon are as yet doubtful. The Archæocyathinæ show affinities with certain corals; a number of tests, included in the genus _Hyolithes_ and its allies are doubtfully referred to the Pteropods, and the position of the genus _Volborthella_ is uncertain. Special attention is directed to these doubtful relationships, as it is possible that a number of 'generalised forms' of organisms occur in these strata[68]. [Footnote 68: For an account of the _Olenellus_ fauna see Walcott, C. D., "The Fauna of the Lower Cambrian or Olenellus Zone," _Tenth Annual Report of the Director of the United States Geological Survey_, Washington, 1890. It is possible that some of the fossils mentioned in that report belong to strata above that containing _Olenellus_.]