THE ORDERS OF THE MAMMALIA. 103 



extend on to, or over, them. The nasal bones are short or 

 rudimentary, and the lachrymal bones are absent or small. 

 The tympanic bone is thick and scroll-shaped. The mandible 

 has a very small coronoid process, and the condyle is situated 

 at the posterior extremity of the ramus. 



There are no clavicles. There is no complete articulation 

 between the bones of the fore-arm and the humerus, or between 

 those of the carpus, fore-arm and digits. Some of the digits 

 .have more than three phalanges. The pelvis is rudimentary, 

 and there is never more than a trace of hind-limb bones. 



There are two sets of teeth only in the extinct Zeuglodon, but 

 teeth may exist and be replaced before birth by baleen plates. 



In this order the Whale-bone whales, the Dolphins, and 

 the extinct Zeuglodonts are comprised. The Cetacea are closely 

 allied with the Carnivora, among the mammals with zonary 

 placentation. 



Two orders of Mouodelphous Mammalia remain. The 

 placentation of one of them, the SIRENIA, is unknown. 



Like the Cetacea, the Sirenia have a horizontally-flattened 

 caudal integumentary fin, and the hind-limbs and sacrum are 

 absent in existing genera, but in other respects they differ from 

 the Cetacea completely, and approach the Prdboscidea among 

 the zonary and deciduate Mammalia. 



They possess a fleshy snout and lips, a well-developed third 

 eyelid, vesicula3 seminales, and salivary glands, all of which are 

 absent in the Cetacea. 



The teats are thoracic, and two in number. 



The cervical region is short, but longer in proportion 

 than in the Cetacea, though the number of the vertebrae may 

 be only six (Manatus). There is an odontoid process. The 

 heads of the ribs articulate with the centra of all the dorsal 

 vertebrae, which is never the case in the Cetacea. The skull 

 has an enormous zygomatic arch. The premaxillae occupy 

 a large space in the upper boundary of the gape of the mouth ; 

 and the mandible has a large coronoid and high ascending 

 part of the rarnus, in which respeets it is opposed to that of 

 the Cetacea. There are no clavicles. The bones of the fore-limb 



