242 Part III. — Twentieth Annual Beport 



The Averages. 



The average of the percentages for each measurement was calculated, 

 and is to be found in the bottom line of the Table. Owing to the com- 

 paratively small number of fishes which have been examined it has not 

 been considered advisable to calculate the fluctuations of the means of 

 the various dimensions. Tliere is also the further point, that it would 

 have been preferable that the same homologous standard of length had 

 been adopted. In the case of the cod the measurements are from the 

 tip of the premaxilla, in the saithe and lythe from the tip of the 

 mandible. As the averages are used for a means of comparison between 

 the three species as to their external characters, the averages in the case 

 of the saithe and lythe have been corrected so that they represent in 

 Tables VII. and VIII. the distances measured from the tip of the pre- 

 maxilla, converted into percentages of the length of the fish measured 

 from the tip of the premaxilla to the extremity of the middle ray of the 

 tail fin. This operation has resulted in very little change in the saithe 

 quantities, the greatest difl:erence (viz., reduction) not exceeding "5. In 

 the case of the lythe the reduction is as great as 1'2. In five dimensions 

 only is there a reduction of as much as 1, viz., distances of ventral fin, 

 opercular cleft, pectoral fin, anus, and the beginning of the first dorsal fin. 



