350 THE ANATOMY OF VERTEBRATED ANIMALS. 



cetacean. The zygomatic processes of the squamosal are large 

 and thick, and the supraorbital processes of the frontals wide 

 and expanded as in the Cetacea. 



The scapula appears to have had a spine and acromion like 

 that of Manatus. The humerus is compressed from the side, 

 and has true articular surfaces upon 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 diifers from all the other Cetacea in the circum- 

 stance that some of its teeth have vertical successors. 



The Deciduatb M ammat. ta. — These may be subdivided, 

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

 Zonaria and the Discoidea. In the former the placenta sur- 

 rounds 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 disk, which is sometimes more or less lobed. 



Tlie mammalia which possess a zonary placentation are the 

 Gctrnivora, the Prohoscidea, and the Syracoidea. 



Each of these divisions is very closely related to one of the 

 foregoing. Thus the Carnivora approach the Cetacea; the 

 Proboscidea, the Sirenia ; and the Syracoidea, the tingiir 

 lata. 



The ZoNAEiA. 1. The Caenivoea. — In this order the 

 head, relatively to the body, is of moderate or small size ; and 

 hair is abundant. 



The cervical vertebras are free and unanchylosed, and their 

 centra are elongated. The odontoid process of the second is 

 well developed. The dorso-lumbar vertebrae are almost always 

 twenty in number, rarely twenty-one or nineteen. The num- 

 ber of dorsal and lumbar vertebra, respectively, varies be- 

 tween sixteen dorsal and four lumbar, and thirteen dorsal and 

 seven lumbar. The dorso-lumbar vertebriB are always articu- 

 lated together by their zygapophyses, and there is a com- 

 plete sacrum. 



The sternebras are numerous and laterally compressed. 



In the skull the nasal bones are well developed, and have 

 the ordinary form. When supraorbital enlargements of the 

 frontal exist, they are of moderate size. The parietals unite 

 in a long sagittal suture. The orbit and the temporal fossa 

 communicate freely, the posterior boundary of the orbit never 

 being completed by bone. The jugal bone is large and unites 

 by a broad surface with the maxilla. There is a distinct 



