MAD 



[271] 



MAG 



MA'DKEPOEE. (madrepore, Fr. corps 

 marin pierreuX) qui resemble a des 

 rameaux, d une vegetation.) "A 

 stony polypifer, fixed, subden- 

 droidal, ramified ; the surface fur- 

 nished on every part with project- 

 ing, muricated cells : the interstices 

 porous. The cells scattered, dis- 

 tinct, cylindrical, tubular, and 

 prominent, hardly any stella ; the 

 lamellae of the internal parietes 

 very narrow." Parkinson. In a 

 living state, the stony matter is 

 covered with a skin of living gela- 

 tinous matter, fringed with little 

 bunches of tentacula ; these are the 

 polypi: the skin and the polypi 

 contract on the slightest touch. 

 Madrepores are sometimes united 

 and sometimes detached ; where 

 the lamina take a serpentine di- 

 rection they are called meandrina, 

 or brain-stone. 



The term Madrepore is generally 

 applied to all those corals which 

 have superficial star-shaped cavities. 

 Madrepores raise up walls and 

 reefs of coral rocks with aston- 

 ishing rapidity, in tropical cli- 

 mates. 



MA'DBE'POEITE. 



1. Fossil madrepore. 



2. A variety of limestone, found 

 in large rounded fragments, com- 

 posed of numerous, small prisms, 

 nearly cylindrical. Opaque; surface 

 dark brown; fracture conchoidal 

 and black. Constituent parts, car- 

 bonate of lime 63, silex 13, alu- 

 mine 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 com- 

 posed of a soft yellowish-white lime- 

 stone, resembling chalk, and con- 

 taining siliceous masses, ammonites, 

 hamites, hippurites, baculites, &c. 

 The siliceous masses found in these 

 beds are not composed of black 

 flint, but of chert and calcedony. 



The Maestricht beds repose on the 

 upper chalk with flints. Similar 

 beds occur also at Faxoe, in 

 Denmark. Some of the fossils are 

 cretaceous, but none are tertiary. 

 Deshayes has been unable to 

 identify any of the shells of the 

 Maestricht beds with those of 

 the tertiary deposites. 



M'AGAS. A genus of Brachipodous 

 shells. They are equilateral, in- 

 equivalve, one valve convex with 

 a triangular area, divided by an 

 angular sinus in the centre. The 

 other valve flat, with a straight 

 hinge line and two small projections, 

 a partial longitudinal septum with 

 appendages attached to the hinge 

 within. Two species are recorded 

 from the chalk of Norfolk. 

 Lycett. 



MA'GILTJS. A genus of univalve shells 

 belonging to the family Cricosto- 

 mata, according to the arrangement 

 of De Blainville, and to the order 

 Tubulibranchiata of Cuvier. The 

 shell is thick, tubular, and irregu- 

 larly contorted, having a longitudi- 

 nally carinated tube, at first regu- 

 larly 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 crystal- 

 line texture, but, when it has 

 attained its full size, and has 

 formed for itself a lodgment in 

 a coral, it fills up the cavity of 

 the shell with a glassy deposite, 

 leaving only a small conical space 

 for its body ; it continues to accu- 

 mulate layers of this material, so 

 as to maintain 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 vitreous 

 substance. Eoget. Cuvier. Sow- 

 erly. 



MAGNESIA, (magnesie, Fr.) An earth 

 with a metallic basis called magne- 

 sium. Magnesia consists of mag- 

 nesium 61-4, oxygen, 38-6. Mag- 

 nesia is rarely found pure in a na- 



