1870.] ' Porcupine 9 -Expedition Madreporaria. 



297 



Pliobothrus symmetricus, Pourtales. — A specimen of this doubtful coral 

 (which had been described by M. de Pourtales from the results of dredging 

 in from 100 to 200 fathoms) was sent to me by Dr. Carpenter. It came 

 from the cold area, in from 500 to 600 fathoms. 



There is no doubt that this very polyzoic-looking mass belongs to the 

 American type. The tabuloe are hardly worthy to be called such ; and I 

 place the form amongst the Zoantharia provisionally. 



III. The species of Madreporaria belong to genera which do not contri- 

 bute and have not contributed to form coral-reef faunas. None of them 

 are reef-builders ; but all are essentially formed to live where rapid growth 

 and delicately cellular structures are not required. The forms are strong, 

 solid, and large ; and their rapid and repeated gemmation proves that their 

 nutritive processes went on actively and continuously. 



All the species are very much disposed to produce variations ; and this is 

 especially true as regards those which have outlived the long age of the 

 Crag, the glacial period, and the subsequent time of elevations and subsi- 

 dences. The least-variable species are those which are not known on 

 other areas. 



Two of the three species which are common to the West-Indian deep-sea 

 fauna and that of our north-western coasts are also very variable. 



The persistence of Madreporaria from the earlier Cainozoic period to 

 the present time has been an established fact for several years. Some of 

 the forms which are common to the deep sea of the British area and to the 

 so-called miocene of Sicily are still existing in the Mediterranean. None, 

 however, of the species of Corals found in the British Crag are represented 

 in the deep-sea fauna. 



The existence of Mediterranean forms in the North-west British area is 

 in keeping with the discoveries of Forbes. It has, however, a double 

 significance, and bears upon the presence of West-Indian forms on the 

 North-west British marine area. There was a community of species be- 

 tween the Mediterranean and the West Indies in the Cainozoic period, 

 especially of Echinodermata, Mollusca, Madreporaria, and Foraminifera. 

 After the great alterations of the mutual relations of land and sea which 

 took place before the cold affected the fauna of the Franco-Italian seas, 

 this community of species diminished ; but it lasted through all the period 

 of Northern glacialization, and is proved still to exist slightly by com- 

 paring the Algse, the Corals, the Echinodermata, and the Mollusca. 



The presence of two very characteristic Floridan species, and one less so, 

 off the north of Scotland, is particularly interesting, because they all live 

 in the cold area and flourish there, whilst they appear to be less vigorous 

 in the warmer Gulf-stream near Florida. 



It is impossible to fail to recognize the operation of this stream in 

 producing the emigration of these three species, which are essentially 

 American. 



The solidity and the power of gemmation of the corals within the cold area 



