THE SEI WHALE 



225 



10 \z 



Percentdge of Total Length 



14 



Fig. 6i. Sei whale. Variation of measurement No. 2i. Greatest width of skull. 



spot on the anterior part of the ventral groove area. The white area was developed to 

 the maximum in 20 per cent of the whales examined, and was small in 30 per cent. The 

 lateral margins of the white area are usually very indefinite, the white colouring fading 

 gradually into the grey of the dorsal surface and of the mandible. The width of the 

 white area is frequently restricted by encroachment of pigment from the dorsal surface. 

 The dark or pigmented areas spread downwards over the sides of the ventral grooves 

 to a greater or lesser extent, and not only is the line of division indistinct but it is 

 frequently also very irregular so that tongues and irregular shaped areas of pigment 

 extend on to the sides of the white area. In extreme cases, which occur in about 12 per 

 cent of Sei whales, the white area is thus reduced to a narrow median streak of irregular 

 outline. 



At the posterior end of the white area two white streaks occurred in 25 per cent of 

 the whales examined, running outwards and forwards from the middle Ime towards 

 the insertion of the flippers, forming a more or less well defined anchor-shaped mark 

 (PI. XVIII, fig. 2). The formation of the anchor can also be regarded as being produced 

 by encroachment of two tongues of pigment, one on each side, over the posterior part 

 of the white area. In one case the anchor mark was found to be entirely separated from 

 the ventral white area by the joining of the two pigmented tongues, so that a white 

 chevron mark with the apex directed posteriorly was formed (PI. XVIII, fig. 2). In 

 12 per cent of the whales examined, the occurrence of scattered spots and irregular 

 pigmented areas, often of indefinite outline, was noted in the ventral white area. 

 Posterior to the ventral grooves the ventral coloration is some shade of light grey which 

 merges indefinitely and gradually into the darker grey of the dorsal surface. The light 

 grey colour covers the under-surface of the belly, tail and flukes, and in 14 per cent 

 extends for some distance over the flanks and side of the tail. Some white areas occur 



6-2 



