18 SPECIMENS OF CETACEA. 



pair of mandibular teeth are triangular in shape and laterally com- 

 pressed. In the Narwhal the single developed maxillary tooth 

 forms a spirally twisted horn, or tusk, 8 to 10 feet long. In the 

 Porpoise the numerous small teeth are spade like in form, some of 

 which show indications of cusp-like projections. 



In the descriptive part of the Catalogue the skulls of several 

 species are described, e.g. Platauista, Phocsena, Cephalorhynchus, 

 Lagenorhynchus, Delphinus, Tursiops, Steno, where the teeth are 

 numerous in both upper and lower jaws ; in others, as Orca, 

 Globicephalus, the teeth are greatly diminished in number, and 

 are situated at the anterior ends of both jaws ; in Physeter 

 numerous functional teeth are found in the mandible ; in Grampus 

 only three or four teeth are present on each side of the symphysis 

 of the mandible ; in Ziphius and Hyperoodon the fully developed 

 teeth are reduced to a pair at the free end of the mandible, but as 

 they are almost buried in their sockets they can have but little 

 functional value. 



The Tympano-PerioHc Bones. — In Balasna the height of the 

 Tympanic, though not equal to the length, is materially greater 

 than the breadth ; the keel on the lower aspect is very prominent ; 

 the outer surface is divided by a long, wide, oblique groove into two 

 unequal convexities, of which the posterior is much the larger ; the 

 inner surface is convex and striated, and its upper border, where it 

 turns into the tympanic cavity, is nearly horizontal, and at its 

 anterior or Eustachian end is notched, and well above the keel. 

 In B. mysHcetus this border is thicker than in B. australis and 

 hiscayensis, though not so thick and rounded as in Balienoptera and 

 jNIegaptera. 



In Balsenoptera the height is almost the same as the breadth, 

 and each of these is much less than the length. The inferior 

 keel is not so prominent as in IJalaena, and the adjoining surfaces 

 are not so laterally compressed. The groove which separates 

 the outer surface into two unequal convex divisions is short and 

 comparatively narrow. The inner surface is convex, and where it 

 turns round into the cavity is striated, much thicker, and more 

 rounded at the upper nearly horizontal border than in Balsena ; 

 anteriorly it is notched for the Eustachian tube. B. sihhaldi and 

 B. musculus can be distinguished from B. horealis and B. rosirata 

 by their larger dimensions. In B. musculus the inner surface is 

 more convex, the keel is not so sharp, and the adjoining surfaces 

 are not so laterally compressed as in B. sibbaldi. B. horealis has 

 lihe outer surface less convex, the anterior border is not so well 

 ■defined by a groove, the keel is a narrow ridge, the inner surface 

 is more flattened than in B. musculus. In B. rostrata the height is 

 more than the breadth; the two divisions of the outer surface are 

 ;almost equal in antero-posterior diameter. 



In Megaptera the height and breadth are almost equal, the two 

 ■divisions of the outer surface are prominently convex, the inner 

 ^surface is also distinctly convex; the anterior end is blunt and 



