W. XiOOLL 
:34] 
examined within the British area or that the parasites do not occur 
there. Only more extended investigations can determine which is the 
correct view in any particular case. It may so happen that some of 
the fishes in the first list are essentially British and rarely found in 
other localities, while on the other hand it is certain that many fishes 
in the second list are exceeding rarities in British waters, viz.: Mugil 
capita, Naucrates dnctor, Thynnns ;|;ekn»/.s-, Xiphias gladius, and 
Acipenser sturio. 
In the second place it is obvious that the number of parasitic records 
is about equal in the two lists. This circumstance has to be considered 
from several points of view. Firstly there is the fact that in the first 
list most of the records are recent and made according to modern 
principles of identification, while in the second many of the records 
date from a hundred years ago and have not been confirmed in the 
interval. There can be little doubt that several of the latter will be 
found synonymous with better known species. Many of the species 
recorded by van Beneden, for instance, and a number of other observers, 
require careful re-investigation. Secondly a few of the fishes live both 
in salt and in fresh water, the most outstanding examples of which 
are the common eel, the salmon and the sea trout. Some of the 
parasites of these fishes may prove to be entirely fresh-water and not 
marine. Thirdly it is obvious that in the case of some species of fishes 
the number of records in one list is greatly in excess of that in the other. 
This may be ascribed to; 
1. The fish being common in British waters, has been exhaustively 
examined, or alternatively being rare, has been more exhaustively 
examined in other localities. 
2. The intermediary hosts of the various parasites are more 
common in British waters, or alternatively in other localities. 
As examples might be quoted the whiting {Gadus merlangus) which 
displays the most varied Trematode fauna of any British marine fish. 
This is due in part to its wide distribution in British waters and to the 
fact that it has probably been more exhaustively examined than any 
other species. A glance at the records is sufficient to indicate this. 
The pollack (Gadus pollachius) also harbours a fairly varied fauna, 
though much inferior to that of the whiting. This is, no doubt, partly 
due to the fact that it has not been so frequently examined. 
Besides these gadoids two of the flat-fishes show a Trematode 
fauna, in British waters, which is not surpassed in other localities, 
namely the long rough dab (Drepanojjsetfa plalessoides) and the plaice 
