14 BULLETIN NO. VII. 



box, known as the skull. Through openings in the skull the 

 nerves passing from the brain to the organs of sense emerge. 

 When the spinal column is ossified, it is divided into distinct 

 elements or vertebrae. The vertebral column is the central 

 support as well as the axis of the body. Nearly all paired or- 

 gans are symmetrically arranged on either side of it. The 

 bones which constitute the framework of the body and limbs 

 are directly or indirectly attached to it. Of external movable 

 limbs used in locomotion there are never more than four which 

 although variable in position and function may always be recog- 

 nized as a posterior and anterior pair. The heart is ventral, 

 that is, on the same side of the vertebrae as the alimentary sys- 

 tem. 



Vertebrates are of five distinct sorts which constitute the 

 classes Pisces (fishes), Amphibia (frogs and salamanders), 

 Reptilia (reptiles), Aves (birds), and Mammalia (mammals). 

 Of these five groups the first two and the second two are more 

 naturally allied with each other than with any of the other 

 classes. This more intimate relationship is indicated by uniting 

 fishes and batrachians under the common term Icthyopsida and 

 the reptiles and birds under the name Sauropsida. ( The terms 

 BrancMata and Monocondyla are of the same extension as the 

 above, but less convenient. In order to preserve uniformity in 

 the number of sub-divisions, Mammals are sometimes given as 

 the only class under the province Zygencephala co-ordinate 

 with the above.) As distinguished from these groups, Mam- 

 mals possess the following pecularities: 



The female has mammae or glands which secrete a milky fluid 

 to sustain the young, which, after birth, pass through a long 

 period of comparative helplessness. The two halves of the 

 anterior part of the brain (cerebrum ) are connected by a cor- 

 pus callosum. There are two condyles or articulating surfaces 

 at the back of the skull. The lower jaw is composed of a single 

 bone on each side and the ear contains the malleus and incus. 

 The heart has four chambers and a single aortic arch. The 

 diaphragm is perfect and the lungs are freely suspended in the 

 thoracic cavity thus formed. 



The ribs join the sternum. There are no gills at any time in 

 life. -Blood is warm and the red corpuscles unnucleated. The 

 body is covered more or less completely with hair. 



The scientific definition of a mammal would comprise the 

 points above mentioned and others of similar nature, but prac- 

 tically the notion which springs unbidden in our minds when the 



