398 ORTMANN — DISTRIBUTION OF DECAPODS [Aprils, 



Aside from this, a close relation of the western and eastern faunas 

 of the Central American shores is revealed at a glance. 1 Hill says 

 (/. c, p. 267) that the recent faunas of the opposite sides of the 

 isthmus are so distinct that the communication of waters must have 

 belonged to a very remote time, and that there has been probably 

 no communication since Miocene. We may modify this a little 

 and say that the affinities of the Decapod fauna of the Atlantic and 

 Pacific are unmistakable, and that ive have ample and convincing 

 evidence that there must have once been a connection. The scarcity 

 of identical forms (which in part are not beyond doubt), and the 

 demonstrated fact that generally one (or more) species of the re- 

 spective genera replace each other on either side of America, being 

 distinctly different, although closely allied, shows conclusively that 

 this connection cannot have been of a very recent date. I think 

 that the Decapods confirm Hill's opinion that there was no com. 

 munication whatever of both oceans since Miocene time, and we 

 may add that probably the similarity of both faunas is to be referred 

 in most cases to the Eocene and Oligocene interoceanic connection 

 across the isthmus. After this had been closed sufficient time has 

 elapsed to generally change the characters of the once identical 

 Pacific and Atlantic stock and to render them different species, 

 while only a few have preserved their original characters and are 

 to be regarded as identical species. 



Of course it is possible that some of the similarities of the Pacific 

 and Atlantic faunas go back to earlier (Mesozoic) times, when the 



1 In speaking of a close resemblance of these faunas, I wish to avoid being 

 misunderstood. There are cases that show a close resemblance, but this does not 

 mean that both faunas are closely related in all respects; on the contrary, there 

 are other elements on both sides of Central America that are peculiar to only 

 one of them. The Panamic fauna, for instance, contains Indo-Pacific elements 

 and a very peculiar element that 'belongs to the whole western coast of America 

 (from the Western United States to Chili). I have called attention to this 

 element in a former paper (Zool.Jahrb. Syst., Vol. 9, 1896, p. 582 ff.), but I 

 have been entirely misunderstood by von Ihering, who says ( Rev. Mm. Paul- 

 ista, Vol. 2, 1897, P- 379) that m y theory of a migration along this coast is dis- 

 proved by the fact that different faunas succeed each other along this coast from 

 the south to the north. This is quite true, but it does not disprove my theory, 

 since I never meant to say that the whole of the West American fauna has 

 leached these parts by migration from north to south or vice versa. On the con- 

 trary, only a part of it belongs to this category, and there are other components 

 of the West American fauna which came from quite different sources. 



