D OL 



[ 



of limestone, consisting of magne- 

 sian earth 48 parts, and calcareous 

 earth 52 parts. It derives its name 

 from Dolomieu, a French geologist. 

 There are three sub-species. Von 

 Buch maintains that limestone has 

 been converted into dolomite by its 

 proximity to porphyry in fusion, 

 and that the magnesia has been 

 transferred from magnesian miner- 

 als in the porphyry to the lime- 

 stone ; the magnesia being reduced 

 to vapour or gas. Bdkewell. 



The name Zetchstein (from zeche 

 and stein, Germ.) has also been 

 given to dolomite or magnesian 

 limestone. This is a calcareous 

 deposit, of a somewhat variable 

 aspect; it is fossiliferous. The 

 zechstein has not yet afforded any 

 remains of trilobites. It does not 

 appear to be a deposit widely spread 

 over the European area. As yet, 

 it is principally known in Germany 

 and England. Dolomite is gene- 

 rally of a light fawn or yellow 

 colour, and in some parts of a 

 crystalline, in others of a concre- 

 tionary character. It is included 

 in the new red sandstone group, its 

 position being immediately above 

 the coal measures. It is frequently 

 traversed by veins of carbonate of 

 lime, and there are sometimes met 

 with enclosed in it hollow geodes 

 of calcareous spar, with sulphate of 

 strontian and sulphate of barytes. 



DO'LOMITE MA'BBLE. A variety of 

 dolomite of a white colour, occur- 

 ring in small granular concretions ; 

 these concretions are frequently so 

 loosely united as to fall apart by 

 the slightest pressure. 



DO'MITE. A variety of trachyte, and 

 thus named from being found in 

 the Puy de Dome, in Auvergne, in 

 Trance. It has the appearance and 

 gritty feel of sandy chalk. 



DO'NAX. (donax, Lat. oVa, Gr.) 

 Animal a tethys; an equivalved 

 inequilateral bivalve, with a crenu- 

 late margin, the frontal margin 



4 ] DEE 



obtuse; hinge with two cardinal 

 teeth in one valve, one in the 

 other; the lateral teeth one or two, 

 rather distant. The shells of this 

 genus are in general triangular, 

 inequilateral, flattened, truncated 

 before, and wedge-shaped. It is 

 found in sands and sandy mud, at 

 depths varying to ten fathoms. 

 Nineteen species have been de- 

 scribed, six of which have been 

 found in our seas. Several fossil 

 species occur in the neighbourhood 

 of Paris. 



DO'EIS, In the Linnsean arrangement, 

 a genus of gasteropoda, belonging 

 to the class Mollusca. An animal 

 inhabiting a shell ; body, creeping, 

 oblong, and flat beneath; mouth 

 below, on the fore part; vent 

 behind, on the dorsum, surrounded 

 by a fringe ; feelers two and four, 

 retractile, and placed on the front 

 of the upper part of the body. 

 They are all marine, and are found 

 in every sea. In Cuvier's arrange- 

 ment doris is placed in the order 

 Nudibranchiata. 



DO'RSAL. (dorsal, Fr. dorsdle, It. from 

 dorsum, Lat.) Appertaining to the 

 back, as the dorsal fin, the dorsal 

 ligaments, &c. 



DO'ESUM. (dorsum, Lat.) The back : 

 the ridge of a hill is sometimes 

 called the dorsum. In conchology, 

 it generally means the upper surface 

 of the body of the shell, when laid 

 upon the aperture or opening. In 

 the genera of patella and haliotis, 

 the dorsum means the upper convex 

 surface. 



DBE'ISSIMA. A genus of bivalve shells 

 separated from mytilus by Dr. 

 Vanbeneden; it is regular, equivalve, 

 inequilateral ; umbo with a septum 

 in its interior; there are three 

 muscular impressions. The great 

 depth of the anterior side, together 

 with the nearly semilunar figure, 

 distinguish it from mytilus. It 

 has been found fossil in the second- 

 ary and tertiary rocks. Lycett. 



