INTRODUCTIOX. 15 



useful in classification : the fronto-uaso-premaxillary ; the maxillo- 

 pi-emaxillavy rostrum, or beak ; the hard palate and the pterygoids ; 

 the teeth, their absence, or presence, size, form, and numbers ; and 

 the tympano-periotic bones. 



The Fronto-naso-premaxillary Region. — In the Baleen Whales this 

 region is symmetrical. In the genus Balsena the nasals occupy the 

 interval between the upper ends of the right and left premaxillse, and 

 are flattened and horizontal on the dorsal surface. In Balmioptera 

 rimsculus the inner border is raised into a crest, the dorsum is grooved, 

 and slopes gently downwards and forwards ; in B. sihhaldi the nasals 

 are large, dorsum convex slopes rapidly downwards and forwards, 

 outer border is raised into a crest ; in B. borealis the nasals are short, 

 with a low mesial ridge, dorsum faintly concave from side to side, 

 convex from behind forwards, sloping down in front ; in B. rostrafa 

 the nasals are small, convex on dorsum, have no crest mesially or 

 laterally, and dip rapidly down iti front. In Megaptera the nasals 

 are narrow, pointed, ridged mesially, concave laterally ; an iuternasal 

 fissure receives the nasal spine of the frontal, which appears on the 

 surface between the two nasal bones. The nasal ends of the pre- 

 maxillse incline inwards and are bent with an inward convexity. 



In Physeter the region shows an extreme degree of asymmetry. 

 The upper end of the right premaxilla spreads upwards and to the 

 left, and occupies the place of the right nasal ; the left premaxilla does 

 not reach the left nasal ; the anterior nares are displaced, the right 

 somewhat anterior to the left. The maxillae and frontal are raised into 

 a high crest which bounds the bowl-shaped chamber for the spermaceti. 



In Ziphius the conjoined nasals and the premaxillee are expanded 

 and raised as asymmetrical eminences, which overhang like the eaves 

 of a house the anterior nares ; the right nasal and premaxilla are more 

 expanded than the left. 



In Hyperoodon the nasals are concave forward, the premaxillse 

 are expanded asymmetrically, and the anterior surface overhangs 

 the nares ; the striking specific character is the development of 

 the maxillary crests, which are separated from the nasals by a 

 deep cleft. 



In Mesoplodon the nasals are vertical and locked in between the 

 overhanging borders of the premaxillse, which form rough ridges. 



In Platanista the marked specific character is the pair of plate-like 

 maxillary crests which curve inwards and almost meet above the 

 middle of the face. 



In the Delphinidse the nasals do not form a roof for the narial 

 opening, but are short nodular bones which lie behind the opening, and 

 their long axes are almost vertical. The premaxillse are at the sides 

 of the nares and not quite symmetrical, for the left, as a rule, is 

 shorter than the right. In Monodon, Phocaena, Grampus, Lagenorhyn- 

 chus, and Cephalorhynchus the nasal end of the premaxilla is generally 

 convex, or marked by a longitudinal ridge. In Delphinapterus, 

 Orcella, Orca, Globicephalus, Delphinus, Prodelphinus, and Tursiops 

 the nasal end of this bone is either flat or slightlv concave. 



