198 THE CANADIAN NATURALIST. [Vol. YL. 



numerically largest families, namely, the Cephalopods and Grap- 

 tolites which have siiifered most in the way of extinction. 



3. The colonial fauna is related to the second fauna by no more 

 than four species, all Trilobites, and all found in one colony. 



4. On the contrary, the specific connections between the colon 

 ial fauna and the third fauna are represented by ninety-two spe- 

 cies, or eighty-three pei- cent, of the totaf of colonial species. 



5. The same relationships are shown by the general facies of 

 the fossils, irrespective of specific identities. Thus, the last phases 

 of the second fauna are characterised by a predominance of Tri- 

 lobites and by the rarity of Cephalopods and Graptolites. On the 

 other hand, the colonies and the first phases of the third fauna 

 were characterised by the rarity of Trilobites and the abundance 

 of Cephalopods and Graptolites. 



G. These results lead inevitably to the conception that the 

 species of the colonies have been introduced into Bohemia by 

 migration from a foreign area. This conception becomes more 

 certain by a comparison of the colonial fauna with the Silurian 

 fauna of other countries, by which it appears that many colonial 

 species existed in the Lower Silurian series of the British area, 

 that is at a period earlier than the date of their appearance in 

 Britain. 



Y. Pal^ontological Relations between the Colonial 

 Fauna and the Silurian Fauna of Britain. 



The connexions between the Silurian fauna of Britain and 

 Bohemia are two-fold, direct and indirect. The direct connexions 

 are shown by the fact that several of the colonial species of Bo- 

 hemia are found existing in Britain in the " second fauna," i. e. 

 in the Lower Silurian period. The indirect connexions consist 

 in the fact that some of the Lower Silurian species of Britain are 

 found in Bohemia, not in the colonies, but in the third fauna, i.e. 

 in the LTpper Silurian period. 



The following table shows the number of species which are 

 common to the Lower Silurian of Britain, the colonies, and the 

 Upper Silurian of Bohemia, but which are wholly wanting in the 

 Lower Silurian (second fauna) of Bohemia : — 



Cliiernrus himucroJiatus, Murch. 

 SpharexocliHs minis, Beyr. 

 Atrypu reticularis, Linn. 



