GLOSSARY 327 



Mammalia. — The highest class of animals, including the 

 ordinary hairy quadrupeds, the Whales, and Man, and 

 characterized by the production of living young which 

 are nourished after birth by milk from the teats (i/amwce, 

 Mammary glanas) oi the mother. A striking diiference 

 in embryonic development has led to the division of this 

 class into two great groups; in one of these, when the 

 embryo has attained a certain stage, a vascular connec- 

 tion, called the placenta, is formed between the embryo 

 and the mother; in the other this is wanting, and the 

 young are produced in a very incomplete state. The 

 former, including the greater part of the class, are called 

 Placental mammals ; the latter, or Aplacental mammals, in- 

 clude the Marsupials and Monotremes {Ornithorhynchus), 



Mammiferous. — Having mammae or teats (see Mammalia). 



Mandibles, in Insects. — The first or uppermost pair of 

 jaws, which are generally solid, horny, biting organs. 

 In Birds the term is applied to both jaws with their 

 horny coverings. In Quadrupeds the mandible is prop- 

 erly the lower jaw. 



Marsupials. — An order of Mammalia in which the young 

 are born in a very incomplete state of development, and 

 carried by the mother, while sucking, in a ventral pouch 

 (marsupium), such as the Kangaroos, Opossums, etc. (see 

 Mammalia). 



Maxilla, in Insects. — The second or lower pair of jaws, 

 which are composed of several joints and furnished with 

 peculiar jointed appendages called palpi, or feelers. 



Melanism. — The opposite of albinism; an undue develop- 

 ment of coloring material in the skin and its appendages. 



Metamorphic Rocks. — Sedimentary rocks which have un- 

 dergone alteration, generally by the action of heat, sub- 

 sequently to their deposition and consolidation. 



MoLLUSCA. — One of the great divisions of the Animal King- 

 dom, including those animals which have a soft body, 

 usually furnished with a shell, and in which the nervous 

 ganglia, or centres, present no definite general arrange- 



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