260 CETACEA 



this family. Agabelus, from the same deposits, is an apparently 

 allied, but toothless type. 



Family Delphinid^e. 



Teeth usually numerous in both jaws. Pterygoid bones short, 

 thin, each involuted to form with a process of the palate bone the 

 outer Avail of the post-palatine air-sinus. Symphysis of mandible 

 short, or moderate, never exceeding one-third of the length of the 

 ramus. Lachrymal bone not distinct from the jugal. The anterior 

 facet on the periotic (Fig. 96) for articulation with the tympanic 

 deeply grooved ; and the posterior tympanic surface of the same 

 bone comparatively narrow, with its ridge for articulation Avith the 

 free border of the tympanic ill-defined, and situated close to one 

 edge. Transverse processes of the dorsal vertebrae gradually trans- 

 ferred from the arches to the bodies of the vertebrae Avithout any 

 sudden break, and becoming posteriorly continuous serially AA r ith the 

 transverse processes of the lumbar vertebrae. Anterior ribs attached 

 to the transverse process by the tubercle, and to the body of the 

 vertebra by the head ; the latter attachment lost in the posterior 

 ribs. Sternal ribs firmly ossified. External respiratory aperture 

 transverse, crescentic, with the horns of the crescent pointing 

 forwards. 



A very large group, closely united in essential characters but 

 presenting great modifications in details. The different types are 

 mostly so connected by intermediate or osculant forms that there 

 are great difficulties in grouping them into natural subfamilies. 

 Even the formation of Avell- defined genera is by no means satis- 

 factory in all cases. They may, hoAvever, be divided, perhaps 

 artificially, into two groups. 



Group A. — Head rounded, without distinct rostrum or beak. 

 Rostrum of skull about as long as cranial portion. 



Monodon} — Besides some irregular rudimentary teeth, the entire 

 dentition is reduced to a single pair of teeth which lie horizontally 

 in the maxilla, and in the female remain permanently concealed 

 within the alveolus, so that this sex is practically toothless, while 

 in the male (see Fig. 90) the right tooth usually remains similarly 

 concealed and abortive, and the left is immensely developed, attaining 

 a length equal to more than half that of the entire animal, projecting 

 horizontally from the head in the form of a cylindrical, or slightly 

 tapering, pointed tusk, Avithout enamel, and Avith the surface 

 marked by spiral grooves and ridges, running in a sinistral direction. 

 (When, as occasionally happens, both tusks are developed, the 

 spiral grooves have the same direction in each.) Pterygoids very 



1 Linn. Syst. Nut. 12th ed. vol. i. p. 105 (1766). 



