232 DISCOVERY REPORTS 



Sternum. This bone is completely fused, an indication of maturity. It is broad 

 anteriorly, having a width of 85-5 mm. but narrows rapidly to 31 mm. and increases 

 again to 35 mm. The posterior margin is abruptly truncated. There are eight pairs of 

 sternal ribs, of which the first three are attached directly to the sternum itself and the 

 fourth to the cartilagenous xiphisternal plate. The remaining four sternal ribs have only 

 a tendinous connexion with the sternum ; the last of them is attached to a single-headed 

 rib as in Lagenorhynchus. 



Chevron bones, 15. There is, however, a slight doubt as to whether more may not 

 have been present, since loss is easy in more or less decomposed specimens such as 

 mine. 



Teeth. Many of these are missing from the Falkland skull, there have probably been 



seventeen on the right and nineteen on the left of the upper jaw, but some of the 



posterior alveoli are partly obliterated and others may be completely so. In Wilkins's 



21 

 skull the teeth are - on each side. There is a distinct neck at the line of the gum and 



17 



the tips are rounded and slightly rough. In the adult teeth there are signs of wear and 

 almost every one is curved sharply. 



The epiphyses can be easily discerned in the cervical vertebrae and are quite free in 

 the anterior dorsal region. From the tail, fusion has not advanced beyond the posterior 

 side of the nineteenth vertebra. 



It is therefore at least possible that this animal could have attained a greater length. 



SKULL 



The general character of the skull is that of the genus, but compared with P. phocaena 

 there is a greater width across the preorbital region and the rostrum is more acute. The 

 profile of the supra-occipital rises almost at right angles to the foramen magnum and 

 curves forward rapidly until in the region of the interparietal it forms a triangular and 

 almost flat area on the top of the skull. In the young specimen the rise from the foramen 

 is rather less abrupt, but the flattening at the top is quite obvious. In P. phocaena the 

 profile of the supra-occipital rises at about the same angle as that of the immature 

 P. dioptrica, but it curves steadily and gently to the interparietal region which is marked 

 by a small bony eminence. This eminence has indeed a flat top, but in the P. phocaena 

 examined it was only 3 ■ 2-5 cm., whereas in the larger specimen of P. dioptrica it is 

 9-5 X 4-25 cm. These measurements are made as accurately as possible, having regard to 

 the somewhat vague limits of the areas in question (Plate V, figs. 5, 6). 



The descent from the top of the skull to the level of the nasal orifices is very steep in 

 P. dioptrica, even in the immature specimen, and in the adult it is practically vertical 

 until the nasal bones are reached, a distance of about 2 cm. The prenasal protruberances 

 of the premaxillae are rather flatter in P. dioptrica than in P. phocaena. 



The dorsal surface of the rostrum of P. dioptrica is much more flattened than in 

 P. phocaena, so that in the former the rostral parts of the premaxillae are not visible in 



