ORDER IV GET ACE A 83 



The dentition of Cetacea consists, for the most part, of conical, similar, 

 single-rooted teeth, which frequently develop in very great number (as 

 many as sixty in each jaw). Only in the case of the Zeuglodonts and 

 Squalodonts are the two- or three-rooted hind teeth distinguished from the 

 one-rooted front teeth by a laterally compressed crown. The baleen whales 

 have no teeth at all. The male of the Narwhal has a strong tusk, directed 

 toward the front, in the right upper jaw. Dental succession does not occur, 

 even though, in the case of several toothed whales, the second dentition is 

 present in a rudimentary state ; this second set of teeth, however, never breaks 

 through the skin. From the original heterodont dentition a homodont con- 

 dition develops. Homodonty and reduction of teeth begin at the front end 

 of the rostrum. At first the teeth of the upper jaw disappear, then those of 

 the lower jaw become lost, until the former.presence of teeth is only indicated 

 by a dental groove. 



Owing to their aquatic mode of life the Cetacea have undergone a most 

 extraordinary transformation of limb structure, the appendages having become 

 modified into flippers resembling the fins of extinct marine reptiles. The 

 clavicle is lacking, as is also a separate coracoid. The scapula is dis- 

 tinguished by its unusually broad, flat and comparatively short form. The 

 humerus is short and massive, provided on the proximal end with a hemi- 

 spherical head. The distal end is furnished with two facets, forming an 

 obtuse angle, on which the laterally compressed forearm bones fit. These 

 are movable neither on the humerus (except in the case of Zeuglodon) nor on 

 each other. The Cetacea have five digits, with the exception of a few baleen 

 whales, which are remarkable for having lost the third instead of the first 

 digit through specialisation. The phalanges are not articulated, but are united 

 by cartilage and fibrous tissue, and the digits are entirely surrounded by 

 a common layer of skin. They consist of elongated, flattened phalanges, 

 truncated at the extremities, and slightly constricted in the middle. The 

 number of these phalanges in the second and third digits is as many as from 

 nine to fifteen. The hind-limbs and pelvis are either totally degenerate or 

 consist only of small vestiges of the ischium and femur not showing externally. 



A large, horizontal caudal fin, supported not by bony structure but by a 

 compact intertwining of fibres, serves as the chief locomotive organ instead of 

 limbs. Polygonal, calcified plates on the dorsal and ventral fins of certain 

 whales have led to the theory, supported by Kiikenthal and Abel, that 

 these represent the remnants of a bony armour, which could, however, not have 

 served as a complete body covering. 



In spite of all striking similarities between Cetacea and fishes and the 

 marine reptiles, due to environmental influences, the skeleton of this order 

 is fundamentally mammalian and shows no resemblances whatsoever to that 

 of fishes or reptiles. The Cetacea evolved, not from aquatic reptiles, but 

 in all probability from carnivorous, placental land mammals with normal 



heterodont dentition ' ' ' • They frequently attain a gigantic size and are 



either pelagic or live near the shores. 



Fossil Cetacea appear first in the Eocene with the extinct form, Zeuglodon, 

 become more widely distributed in the Miocene and Pliocene, and at this 

 period become much diversified. 



