MAC 



C 153] 



M A G 



with appendages which most frequently 

 form a fin on each side. This tail is 

 always composed of seven distinct seg- 

 ments. Fossil genera of the family Ma 

 croura have been found in the Muschel- 

 kalk and in the lias. 



MACROU'ROUS. Belonging to the family 

 Macroura. Five or six genera of Ma- 

 crourous decapods have been recently ob- 

 served in the muschel-kalk. 



MA'CTRA. (juajcrpa, Gr. mactra, Lat. a 

 kneading-trough.) Animal a tethys. A 

 genus of equivalve, inequilateral, trans- 

 verse bivalves, slightly gaping at the 

 extremities; the hinge, or middle tooth, 

 complicated ; lateral teeth rather remote, 

 compressed, and inserted. Shells of this 

 genus have only been found to inhabit 

 the ocean, at depths varying from 

 to twelve fathoms, in sands and sandy 

 mud. The French naturalists divide 

 Mactra into two genera, Mactra and Lu- 

 traria. In Turton's Linne twenty-seven 

 species are described ; twelve are inhabi- 

 tants of our seas. The fossil species 

 belong to the tertiary formations. La- 

 marck describes one species, Mactra 

 semisulcata, found in the neighbourhood 

 of Paris ; and Parkinson mentions one, 

 found in Essex. 



MA-CULA. (macula, Lat. macule, Fr. ma 

 cula, It.) A spot, generally of a dark 

 colour. 



MA'CULATED. Spotted ; marked with small 

 spots. 



MA'DREPORE. (madrepore, Fr. corps marin 

 pierreux qui resemble a des rameaux, d. 

 une vegetation.} Madrepores are stony 

 polypi, with concentric laminae, resem- 

 bling stars. In a living state, the stony 

 matter is covered with a skin of living 

 gelatinous matter, fringed with little 

 bunches of tentacula ; these are the po- 

 lypi : the skin and the polypi contract 

 on the slightest touch. Madrepores are 

 sometimes united and sometimes de- 

 tached ; where the laminae take a serpen- 

 tine direction they are called meandrina, 

 or brain-stone. BaJcewelVs Introduction 

 to Geology. 



The term Madrepore is generally ap- 

 plied to all those corals which have super- 

 ficial star-shaped cavities. In the water, 

 madrepores are invested with a fleshy 

 covering of various colours ; when the 

 animal dies this fleshy gelatinous covering 

 becomes decomposed, and the axis ap- 

 pears studded over with lamellated cells, 

 or stars. Madrepores raise up walls and 

 reefs of coral rocks with astonishing rapi- 

 dity, in tropical climates. 



MA'DREPORITE. 



1. Fossil madrepore. 



2. A variety of limestone, found in large 

 rounded fragments, composed of numerous , 



small prisms, nearly cylindrical. Opaque ; 

 surface dark brown ; fracture conchoidal 

 and black. Constituent parts, carbonate 

 of lime 63, silex 13, alumine 10, oxide of 

 iron 11. 



MAE'STRICHT BEDS. The name given to 

 the uppermost member of the cretaceous 

 group, from Maestricht, a town of the 

 Netherlands. The Maestricht beds are 

 marine, and composed of a soft yellowish- 

 white limestone, resembling chalk, and 

 containing siliceous masses, ammonites, 

 hamites, hippurites, baculites, &c. The 

 siliceous masses found in these beds are 

 not composed of black flint, but of chert 

 arid calcedony. The Maestricht beds re- 

 pose on the upper chalk with flints. M. 

 Deshayes has been unable to identify any 

 of the shells of the Maestricht beds with 

 those of the tertiary deposites. 



MAGI'LUS. A genus of univalve shells be- 

 longing to the family Cricostomata, ac- 

 cording to the arrangement of De Blain- 

 ville, and to the order Tubulibranchiata 

 of Cuvier. The shell is thick, tubular, 

 and irregularly contorted, having a longi- 

 tudinally carinated tube, at first regularly 

 spiral, and then extending itself in a line 

 more or less straight. The young of the 

 genus Magilus has a very thin shell of a 

 crystalline texture, but, when it has at- 

 tained its full size, and has formed for 

 itself a lodgment in a coral, it fills up 

 the cavity of the shell with a glassy de- 

 posite, leaving only a small conical space 

 for its body ; it continues to accumu- 

 late layers of this material, so as to main- 

 tain its body at a level with the top of the 

 coral to which it is attached, until the 

 original shell is quite buried in this vi- 

 treous substance. Roget. Cuvier. Sow- 

 er by. 



MAGNE'SIA. (magnesie, Fr.) An earth 

 with a metallic basis called magnesium. 

 Magnesia consists of magnesium 61'4, 

 oxygen, 38*6. Magnesia is rarely found 

 pure in a native state. It enters into the 

 composition of some of the primary rocks, 

 to which it usually imparts a saponaceous 

 feel, producing also a striated texture, 

 and frequently a greenish shade. Mag- 

 nesia first became known about the be- 

 ginning of the eighteenth century. Little, 

 however, was known concerning its na- 

 ture, till Dr. Black published his cele- 

 brated experiments on it in 1755. Mag- 

 nesia may be thus prepared : sulphate of 

 magnesia, a salt, composed of magnesia 

 and sulphuric acid, is to be dissolved in 

 water, and half its weight of potass added. 

 The potass having a stronger affinity for 

 the sulphuric acid than the magnesia has, 

 seizes the sulphuric acid, and the mag- 

 nesia is precipitated. Magnesia is often 

 present in the chalk ; some of the strata 

 x 



