184 C. H. Ostenfeld: 



at a time when the isthmus of Panama was not finally formed. 

 From geological evidence, it is usually agreed that the isthmus is 

 of Tertiary age. and the differentiation of the Caribbean species 

 is consequently more recent. The close resemblance of each species 

 to an Indopacific one agrees very well with this supposition. 

 Their i-estricted geographical areas may be similarly explained. 



Two of them have reached the Bermudas, and this migration 

 accords with the direction of the Gulf Stream drift, which has 

 a very strong flow from the coast of Florida towards the north- 

 west, washing on its way the shores of the Bermudas. With this 

 exception, none of the Caribbean species has yet become distributed 

 in the Atlantic outside the Caribbean region. Although it may 

 be admitted that our knowledge of the sea-coast flora of South 

 America is imperfect, it is hardly likely that any rich sea-grass 

 vegetation, if existing, could have remained unreported.! 



Besides the 4 Caribbean species having their Indopacific partners, 

 two species of Halophila also occur in the Caribbean region, one 

 in the northern part (Florida and Bahamas), and the other in the 

 southern parts (Antilles to Pernambuco). They are very closely 

 allied, and must have come from a common ancestor. On the other 

 hand, they are quite distinct from other species of Halophila, and 

 no allied species has yet been found in the Indopacific region. It 

 is possible that such a form may yet be found, but it is also pos- 

 sible that it has had its day and then died out there, and that the 

 Caribbean forms are the only remnants now existing of this par- 

 ticular section of the genus. 



V. The Australian groujj. I have referred 5 species to thig 

 group, but it is not so homogeneous as I could wish : 



Cymodocea antarctica. 



Posidonia australis. 



Zostera capricorni. 



Zostera Mueller i. 



Zostera tasmanica. 



The systematic value of the three species of Zostera is somewhat 

 uncertain. It is possible that they ought to be reduced to two. and 

 even one of these might perhaps be united with Z. nana. Their 

 area of occurrence is along the east coast of Australia, from Cape 

 York southwards to Tasmania, and along the eastern part of the 



1. Anioiipst the sea-jfrasses on record from the -Afrie^an .side of the Atlantic, there is a sterile 

 species of Diplantlwia, found at Loanda (Guinea). Whether it is the Caribbean D. Wrightii or, 

 more probably, the Indopacific D. uninervig, already recorded from the East-coast of Africa, it 

 is impossible to say. 



