4 G. P. MATTHEW ON THE 



compare that distant time with the present. In the many ages which have since elapsed 

 there have been great changes in the physical constitution of the earth, and the beings of 

 to-day bear but a remote resemblance to those which swarmed in the Cambrian seas. 

 Nevertheless the differences are not so great, nor the earlier animals so diverse in structure, 

 as entirely to preclude comparison with those now existing on the earth. 



Then as now there were species that clung to the shores and shallows of the ocean, 

 and others adapted to life in the open sea distant from any shore. Then as now there 

 were species that harboured in the mud which gathered at the bottom of sheltered bays, 

 and others that delighted in the rolling surf of an exposed coast. On the one hand there 

 were species that migrated rapidly from one part of the ocean to another ; on the other 

 there were forms which were retarded or wholly prevented from migrating to distant 

 parts of the ocean, either by their dependence on the shores and shallows for sustenance 

 and reproduction, or by their feeble locomotive powers. 



Among marine organisms of that early time, which from the rapidity with which they 

 spread from one part of the ocean to another, and from their almost world-wide distribu- 

 tion, are of value in determining exactly the sequence of the minor divisions of the 

 geological scale in Palaeozoic times, the first rank may be conceded to the graptolites. 



Like their congeners the Sertulariau Zoophytes, these animals seem to have beeii 

 capable of reproduction in the open sea, distant from any shore, and so fitted for rapid 

 propagation to distant parts of tlie ocean. This group therefore has been selected as the 

 standard to which others may be referred in determining their age and rate of 

 dispersion.' 



Within the Cambrian system and the lower part of the Ordovitian. without consider- 

 ing minor changes, there are four principal graptolitic faunas. The first of these is found 

 in the Lower Cambrian division, but the structure of the species thus far known is quite 

 obscure, and the hydrosomes fragmentary. In Sweden only " spores " (Siculœ ?) of grap- 

 tolites are known in this part of the Cambrian system." In America beside a Dendro- 

 graptus, some obscure alate forms are known, as Protograptus from the highest Para- 

 doxides horizon of the St. John G-roup (Div. Id) and Phyllograptus ? ? and Cliraaco- 

 graptus ? ? from the Oleuellus horizon in Vermont. 



At the second graptolitic horizon, which is in the upper part of the Cambrian system, 

 the species are in a better state of preservation, and have been carefully studied. At this 

 level occurs Dictyonema ftabeHiforme and a few introrse '* Dichograptids. 



The third graptolitic horizon, which is at the base of the Ordovician system, is chiefly 

 remarkable for the great abundance of introrse Dichograptids, to which Phyllograptus 

 was added. 



' At some future time the hexactinellid sponges may be found useful for this purpose, being lilce the graptolites 

 more or less pelagic in their habits ; but up to the present time the Cambrian species are very imperfectly known, 

 and we must be content to depend upon the graptolites for the exact definition of geological horizons in early 

 Palfeozoic time. The admirable work done l>y Sir .1. Wm. Dawson and Mr. G. J. Hinde on the sponges found in 

 the slate rocks at Metis on the Lower Si. Lawrence, opens up a new world of forms in this group of animals. Mr. 

 Walcott had already illustrated the genus Cyathospongia from material found in the Utica slate, but Sir William 

 has described a variety of genera (of which some will no doubt be found to characterize special horizons) and 

 thus given us new points of departure for Palfeozoic investigation. 



' Die Silurischen Etagen 2 und 3. 



■'■ Those in which the cells are turned inwards, towards each other, on the branches. 



