CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 345 



GROUP 3. MAMMALIA. 



Class 1. Mammalia. Warm-blooded Amniota in 

 which an epidermic exoskeleton, in the form of hair, 

 is always present at some period of life. The vertebrae 

 possess epiphyses, and the cervical vertebras are 

 usually seven. The occipital region is well ossified, 

 and there are two occipital condyles. The otic bones 

 early fuse together into a periotic. The mandible is 

 of two pieces only, and articulates directly with the 

 squamosal. The lips are usually definite and muscular, 

 and the teeth, when present, are only replaceable to a 

 limited extent by new ones. The large intestine is 

 very long, and usually opens independently, behind a 

 urinogenital aperture. The red corpuscles are non- 

 nucleated, the heart four-chambered, the right auriculo- 

 ventricular valve membranous, and there is a single 

 aortic arch, curving to the left. There is an epiglottis, 

 the lungs are mobile, there are no air-sacs, the bronchial 

 tubes end in infundibula, and there is a complete respi- 

 ratory diaphragm. The cerebral hemispheres are united 

 by a corpus callosum, a third commissure and corpus 

 mammillare are present, and the optic lobes are double. 

 The sclerotic coat of eye is fibrous, there are three 

 auditory ossicles, and the cochlea is (almost always) 

 spirally coiled. Usually viviparous, and the young 

 always nourished by milk for some time after birth. 



Sub-Class 1. Prototheria (Ornithodelphia). Mam- 

 mals devoid of functional teeth, and possessing 

 distinct coracoids, epicoracoids, interclavicle, and 

 epipubic (marsupial) bones. There is a cloaca, the 

 young are hatched from large eggs with a calcareous 

 shell and much food-yolk, the cleavage being conse- 

 quently meroblastic. The mammary glands have no 

 teats. 



Order 1. Monotremata. Ornithorhynchus, Echidna, 

 Proechidna. 



The remaining sub-classes of Mammals include vivi- 

 parous animals which usually have functional teeth, 



