230 DISCOVERY REPORTS 



and there is an unpigmented band extending from the blow-hole to the rostrum, a 

 feature absent in the larger animals. There is also a good deal of colour on the ventral 

 surface of the flukes. 



Wilkins's specimen is described as having been blue-black above and dirty white 

 below. In it the mandibulo-pectoral band showed clearly, and apparently the tail is as 

 dark or nearly as dark below as above. This animal shows a coloration resembling that 

 of the foetus, and it is reasonable to assert that the pale or uncoloured areas become more 

 extensive with age and that the colours become brighter. It may well be that similar 

 transitions are to be found in other dolphins. This specimen is noted by the collector 

 as ? juv. 



The skull of Wilkins's specimen is clearly that of an immature animal. There is a 

 general lack of development and the teeth have scarcely if at all erupted. The Falkland 

 skull is much developed and the teeth stand well up from the gum. 



SKELETON 



There is, unfortunately, no description of the skeleton of any of the three Argentine 

 specimens, so that the following notes are based on Wilkins's immature and my adult 

 specimens. Comparisons have been made with P. phocaetia, the best known species of 

 the genus, and P. spinipinnis as described and figured by Allen (1925). 



Vertebral coliumi. Cervical vertebrae, 7. Of these the first five are fused and the 

 sixth and seventh are free, whereas in P. phocaena only the seventh is free. The whole 

 series is extremely compressed antero-posteriorly and possesses marked bilateral 

 asymmetry. 



The neural arch is incomplete in the fifth and seventh vertebrae but complete in the 

 sixth. The neural spine of the atlas is deeply cleft, so that the fifth, sixth and seventh 

 vertebrae are visible in dorsal aspect and it only partly embraces the seventh neural 

 spine. In P. phocaena this spine covers the remaining cervical vertebrae to a much 

 greater extent (Plate VI, figs, i, 2, 3, 4). 



Vertebrarterial canals are present in the fifth vertebra, complete on the right side and 

 incomplete on the left. 



Dorsal vertebrae, 13. The height of the neural spines increases until the seventh is 

 reached and therefore decreases towards the tail. The first and second spines have sharp 

 ends, whereas those of P. phocaena are rounded. The transverse process of the first 

 dorsal vertebra exhibits a slight ridge on the antero-dorsal aspect, and if examination is 

 made of the corresponding region in successive vertebrae this ridge is found to assume 

 a more and more central position until on the twelfth vertebra it forms a well-marked 

 prezygapophysis herein agreeing with P. phocaena (but not with P. spinipinnis). All the 

 neural spines slope backwards, the greatest inclination being attained by the sixth. 



Lumbar vertebrae, 16. The neural spines attain their greatest development at the 

 fifth and sixth lumbar which are about the same size. These spines become more and 

 more erect towards the tail, but there is never the slightest indication of the forward 

 curvature characteristic of P. phocaena. In this P. dioptrica agrees with P. spinipinnis. 



