DIFFUSION OF THK CAMBRIAN FAUNAS. S 



At the fourth horizon extrorse Dichograptids appear and become numerous. These 

 are spread through a considerable thickness of rocks, and two principal groupings of 

 species are distinguished by Prof Lapworth ; of these the older is marked by the presence 

 of the genus Cœuograptus, and the later by its absence. 



These four graptolitic zones may be used as chronological tests for determining the 

 comparative age of the littoral faunas. 



II. — Other Deep-w.^ter Organisms. 



With these graptolites occur species of deep-water sponges, of trilobites and of 

 brachiopods that were able to maintain their existence without recourse to the shores of 

 the ocean. These on account of their similar habits may be used in connection with the 

 graptolites, in determining the .sequence of fossili ferons horizons. 



Of the sponges, a Hexactinellid, Protospougia, is the genus best known. This both 

 in Sweden and Eastern Canada was contemporary with the graptolitic fauna of the Para- 

 doxides beds, and evidently flourished in a similar habitat. It may therefore be counted as 

 a deep-water form ; but unfortunately, up to the present time, it is known only by frag- 

 ments of its skeleton, and its general form is as yet iinknowu. Single cruciform spicules 

 which have been found at higher horizons in the Cambrian system have been assigned to 

 Protospougia, but such, in thi> light of our present knowledge, can only be regarded as 

 indicating a Lyssakiue sponge, and not necessarily the genus Protospougia. 



Associated with these graptolites and hexattinellid sponges there are certain trilobites, 

 which seeing that they are found in company with deep-water forms may be included in 

 the open-sea faiina. These, however, are in general not so characteristic, so far as the 

 genera are concerned, as are the graptolites and sponges ; for in some cases we find species 

 of the same genus of trilobites in deposits which were made in shallow water or near shore 

 lines, and others that belonged to deeper waters, but in these, though the genera are thus 

 diffused, the species are often definite in their habitat. With the first graptolitic fauna 

 certain species of Paradoxides, Microdiscus, Solenopleura, Agnostus, etc., thus occur. 



Associated with the second graptolitic fauna are the genera Peltura and Parabolina, 

 and the family of the Leptoplastidœ, all of which appear to have favoured a habitat where 

 soft mud accumulated in still and comparatively deep water. With the third graptolitic 

 fauna were thetrilobitic (sub-genera) Cyclognathus and Parabolinella, and with the fourth 

 Triarthrus. 



Among the brachiopods also there were certain species of the genera Linnarssonia 

 Obolus, Lingulella and Acrotreta which are found associated with the graptolites and 

 therefore capable of living in deep water ; while other species of these genera belon"-ed 

 to the shallows and sandy shores. 



Graptolites are organisms which are not very readily detected in the rocks ; often 

 they occur only in thin bands which are easily overlooked ; but the remains of animals 

 of the littoral faunas are often abundant and diffused through a considerable thickness 

 of rock ; hence they are apt to be used first for determining the age of rocks. But as the 

 necessity for a close comparLsou of faunas indifferent parts of the earth becomes more 

 imperative the graptolites and their associated faunas will be more highly prized for the 

 exact demarkation of geological horizons. 



