300 CTEXOPHOR.E. Part II. 



deposits of Bohemia, is to introduce the element of time in the deflnition of fauna. 

 Avhieh is foreign to it.^ If the exjiression of " first fauna "' could be made to mean 

 all the animals of that epoch, then we should in the same way be justified in 

 speaking of the present fauna as including all the animals now living upon earth ; 

 while it is well known that there are numerous faumv at present ranging over our 

 globe, as there no douljt have been at all times. We may speak of the "first era" or 

 " first period " in the development of animal life npon earth, but certainly not of the 

 '•first fauna;" though we may say that the '-Bohemian fauna" of the first geological 

 era has Jjeen described in a masterly manner by Barrande. In so doing we shall 

 avoid confounding geological periods with geographical areas. In the same way will 

 it be necessary to distinguish Ijetwcen the French, the German, and the English faunte 

 of the different geological horizons of the Jurassic and of the Cretaceous series. We 

 shall have a German and an English fauna of the Lias period, a Swiss and a 

 French fauna of the Xeocomian period, etc.. etc., as soon as the natural boundaries 

 of all these fauna"", for all the successive geological epochs, have been satisfactorily- 

 traced. And the surest method to advance the solution of this problem is, un- 

 questionably, to distinguish carefully the diflerent elements of the question before u.s, 

 and not to confound time and space. 



These distinctions being admitted, we may now proceed to consider the Aca- 

 lephian fauna> of the present period, as characterized l)y the Ctenophonv. I have 

 already alluded to the relations noticed between the representatives of the different 

 liimilies of this order, and the physical conditions under which they live. It remains 

 to examine their condjinations in diflerent zoological provinces. 



The arctic regions ha\ing been scantily explored, as far as the Acalephs are 

 concerned, it can hardly jje expected that much shovdd be known of the fauntc of 

 this zone; and yet it already appears, from the oltservations of Martens, Scoresljy, and 

 Mertens, that the northernmost parts of the globe are inhabited by Acalephs which 

 differ from those of the boreal zone. Mertensia Cucullus and M. depressa are the 

 two most northern Ctenophora' known; and if these two species prove to be really 

 distinct, it would follow, that the Atlantic side of the Arctic Ocean, on which Idyia 

 borealis is found with Mertensia Cucullus. forms a distinct founa from that of the 

 Pacific side, where Dryodora ghmdiformis accompanies Mertensia compressa. 



In the boreal zone we nuiy already distinguish three fauna^ : 1°, a Scandinavian 

 fauna; 2% an Acadian fuuia; and, 3", a Columbian fauna. The Scandinavian fauna is 

 characterized Ijy Bolina norvegica, Pleurolu-achia l.)icolor, and Idyia Cucumis; the Aca- 



1 ISnriTmdc luis alreaJy slitiwn tliat " the range in the dii-ection from Sweden to Bohemia, than 

 of (li>lriluilinn of tlie Trilohites during tlic period is the ea^e witli tlje living Crustacea." ParallMe 



of tln-ee .siieeessive sihirian fauna\ was more limited entre les depots siluriens. etc.. p. C7. 



