DESCRIPTION OF FISH 307 



To insure this, the safest plan is to preserve at least 

 one specimen of such fish ; but as any individual, 

 however carefully preserved, is almost certain to under- 

 go some post-mortem change of shape, and to lose 

 the colours which adorned it in life, it is well, wherever 

 possible, to take careful measurements, and make a 

 sketch or note of the colours of the living or fresh 

 animal. This is important, both in the case of colours, 

 wJiich frequently differ in the two sexes, and are nearly 

 always subject to change at or shortly after death ; 

 and also in the case of delicate scales or fins, which are 

 easily damaged. 



Fig. 203 is intended to show the more salient points 

 in the external topography of a fish, and the names by 

 which they are commonly referred to in descriptive 

 literature. 



In taking the measurements of a fish the accepted 

 method is, not to follow the contours of the body, but to 

 measure between perpendiculars, as shown in Fig. 204. 

 The measurements ordinarily used in descriptions are 

 those there shown, and, in addition, the distance be- 

 tween the eyes (measured by callipers or compasses 

 between their bony orbits), and sometimes the greatest 

 bieadth of the head and body respectively. These 

 measurements should be taken in centimetres and 

 millimetres. 



In addition to these measurements it is usual in 

 the case of Teleostean fishes to count the number of 

 rays in the dorsal and anal fins, discriminating 

 between spines and soft rays, and the number of 

 scales (when present) in a longitudinal series along 

 the body above the lateral line and in a transverse 

 series from the back to the belly at the deepest part 

 of the body. 



In cases where there is more than one dorsal or anal 



