260 CETACEA 
this family. Agabdelus, from the same deposits, is an apparently 
allied, but toothless type. 
Family DELPHINID.£. 
Teeth usually numerous in both jaws. Pterygoid bones short, 
thin, each involuted to form with a process of the palate bone the 
outer wall 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 with the 
free border of the tympanic ill-defined, and situated close to one 
edge. Transverse processes of the dorsal vertebre gradually trans- 
ferred from the arches to the bodies of the vertebre without any 
sudden break, and becoming posteriorly continuous serially with the 
transverse processes of the lumbar vertebra. 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 well-defined genera is by no means satis- 
factory in all cases. They may, however, 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, without enamel, and with 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. Nat. 12th ed. vol. i. p. 105 (1766). 
