CHAP. IV.] THE CRUISES OF THE 'porcupine: 177 



cation of structure under altered conditions of life. 

 Three specimens were procured on two different 

 occasions, and they are in all respects similar." 



MoUusca are much, more abundant and varied 

 in the warm area than in the cold. Mr. Gwyn 

 Jeffreys remarks, however, that there is not such 

 a decided difference in the MoUuscan fauna of the 

 two regions as might have been expected from the 

 difference in their conditions ; very many species 

 being common to both. At 500 fathoms the sponges 

 are full of Tecten vitreus, Chem., and Columbella 

 halioeti, Jeffreys ; and throughout the area species 

 occur of many MoUuscan genera, including Lima, 

 Dacridium, Nucula, Leda, Montacuta, Axinus, Astarte, 

 Tellina, Necera, Dentalium, Cadulus, Siphonoden- 

 talium, jRissoa, Aclis, Odostomia, Aporrhais, JPleuro- 

 toma, Fusus, and Buccinum. 



Taken as a whole the fauna of the warm area off 

 the north of Scotland seems to be an extension of 

 a fauna with which we are as yet very imperfectly 

 acquainted, occupying what we must now call 

 moderate depths, say from 300 to 800 fathoms, 

 along coasts which are bathed by currents of equa- 

 torial water. The fauna of this zone is evidentlv 

 extremely rich ; and as it is beyond the reach of 

 ordinary dredging from an open boat, and yet not 

 at a suflB.cient depth to present any very great diflBl- 

 culty from a yacht of average size, its exploration 

 seems to present just the combination of adventure 

 and novelty to stimulate amateurs; so we may 

 hope shortly to have its conditions and distribution 

 cleared up. A most successful step in this direction 

 has been made already by Mr. Marshall Hall, who. 



