374 ON THE FAUNA AND BOTTOM-DEPOSITS NEAR THE 30-FM. LINE 



above, and upon the wandering animals which entered it from other 

 grounds. The nature of the larval forms would depend upon the 

 nature of the grounds over which the water had previously passed, 

 and it would appear that the supply of larvai from grounds in the 

 immediate neighbourhood would be much greater than that from those 

 which were more distant. In this connection the tidal current, running 

 alternately in opposite directions, will evidently be an important factor. 

 It will be shown in the present report that in many cases the nature of 

 the fauna at a particular spot is distinctly intiuenced by the fauna of 

 grounds in the immediate neighbourhood. 



The principle just discussed may be extended to the fauna of any 

 particular district, which will be influenced by the nature of the fauna 

 in surrounding districts. The fact, which has long been well known, 

 that the fauna in the western portion of the Channel is composed of a 

 mixture of northern and southern species is, to some extent, a result of 

 the principle enunciated. 



Section III. 



THE rilYSICAL CONDITIONS PIIEVAILING ON THE 

 GEOUNDS INVESTIGATED. 



The grounds have been so chosen that, with the exception of changes 

 in the nature of the bottom-deposit, the differences in their physical 

 conditions are probably only slight and not of great importance. Under 

 the present heading those conditions, which are nearly the same for all 

 the grounds, will be described, whilst the nature of the changes which 

 the bottom-deposit undergoes will be reserved for an independent 

 section. 



The grounds lie in the immediate neighbourhood of the 30-fathom 

 line (Charts I. to XVI ), all of them being between the 25- and 35- 

 fathom lines. In the neighbourhood of the Eddystone the 30-fathom 

 line is from eight to nine miles distant from the coast, and within about 

 a mile of the exposed Eddystone rocks. Near Bolt Head and Prawle 

 Point, on the other hand, it approaches to within 1^ miles of the 

 shore. 



The Composition of the Water. The density of the water on all the 

 grounds may be taken, for the purposes of the present inquiry, to be 

 that of normal Atlantic water, any variation from this figure being so 

 small as to be without importance in. influencing the distribution of the 

 bottom fauna. 



This appears to be the case even for the Bolt Head and Prawle 

 Grounds, which lie close to the entrance of the Salcombe estuary, as is 



