THE DECIDTJATE MAMMALIA. 411 



is devoid of a bone, and, ordinarily, is bent up in tlie long 

 preputial slieath. 



c. The Pliocodontia are represented only by Zeuglodon, 

 Squalodon, and other large extinct cetaceans of the tertiary 

 epoch. These remarkable fossil forms constitute con- 

 necting links between the Cetacea and the aquatic Carni- 

 vora. The cervical vertebrae are distinct and unanky- 

 losed, nearly resembling those of the Rhyncoceti. The 

 caudal vertebrae have theii* tx'ansverse processes perforated 

 vertically, as in many Cetacea. The distal ends of the 

 I'ibs are enlarged somewhat as in the Sirenia. The skull 

 is symmetrical, and the nasal bones, though still short, are 

 longer than those of any other cetacean. The zygomatic 

 processes of the squamosal are large and thick, and the 

 supraorbital processes of the frontal s wide and expanded 

 as in the Cetacea. 



The scaj)ula appears to have had a spine and acromion 

 like that of Manatus. The humerus is compressed from 

 side, and has true articular surfaces iipon its distal end, 

 although they are of small size. 



The molar teeth have laterally compressed crowns with 

 serrated edges and two fangs, resembling those of many 

 seals, and Zeuglodon differs from all the other Cetacea in 

 the circumstance that some of its teeth have vertical suc- 

 cessors. 



The Deciduate Mammalia. — These may be subdivided 

 according to the form of the placenta, into two groups : 

 the Zonaria and the Discoidea. In the former the placenta 

 suiTounds the chorion like a hoop, leaving its ends free of 

 villi, or nearly so. 



In the Discoidea, on the other hand, the placenta takes the 

 form of a thick disc, which is sometimes more or less lobed. 



The mammalia which possess a zonary placentation are 

 the Carnivora, the Proboscidea, and the Hyracoidea. 



Bach of these divisions is very closely related to one of 

 the foregoing. Thus the Carnivora apjjroach the Cetacea ; 

 the Proboscidea, the Sirenia ; and the Hyracoidea, the 

 JJngulata. 



