L E P 



[255 ] 



L I A 



ofiishes inhabiting the rivers of 

 North America, one of the two 

 living representative genera of 

 Sauroid fishes. Teeth of a fish 

 related to Lepidosteus, or Lepisos- 

 teus, have been found in the Til- 

 gate beds and in those of Stones- 

 field. 



LEPIDO'STEOBUS. | A genus of extinct 

 LEPIDO'TUS. j fresh water fishes, 

 with very thick, enamelled, rhom- 

 boidal scales, and obtuse hemi- 

 spherical teeth; the latter are 

 called fishes' eyes by the common 

 people who collect them. 

 LEPTJE'NA. (from XeTrros, tenuis, Gr.) 

 A sub-division of the family of Te- 

 rebratula, established by Dalman. 

 The name Leptsena may be deemed 

 synonymous with Productus or 

 Producta, but the latter being ob- 

 jected to, it has been thought de- 

 sirable to adopt that of Leptaena 

 instead. Some authors separate 

 Leptsena from Productus from the 

 character of the hinge, which is 

 compressed and rectilinear, fre- 

 quently exceeding the width of 

 the shell. See Producta. 

 LET/CIN. | (from \CVKOS, white, Gr.) 

 LET/CTNE. ) The name given by M. 

 Braconnot to a white substance 

 obtained from muscular fibre, by 

 treating it with sulphuric acid, and 

 subjecting it to a peculiar process. 

 LEI/CITE. (from XevAcos, white, Gr.) 

 A mineral of a white colour, found 

 in volcanic rocks. Notwithstand- 

 ing its name, leucite is not always 

 white, its common colours are 

 yellowish and greyish white, but it 

 is occasionally of a reddish white, 

 and translucent. Before the blow- 

 pipe it is infusible, a circumstance 

 which serves to distinguish leucite 

 from garnet and analcime ; when 

 mixed with borax it fuses into a 

 brownish diaphanous glass. Its 

 constituents are silex 53'75, alu- 

 mine 24*62, potash 21'35; specific 

 gravity from 2'45 to 2-49 ; hard- 

 ness = 5-5. It scratches glass 



with difficulty. It occurs regularly 

 crystallized ; in granular concre- 

 tions, and in roundish grains. It 

 is often embedded in lava and in 

 basalt. All lavas do not contain 

 crystals of leucite. In the lava of 

 Vesuvius they are abundant, but in 

 that of Etna they are rarely found. 



LEFCO'NIDA. An order of the class 

 poriphera, thus named from the 

 body being of a white colour, the 

 spicula of the skeleton being also 

 calcareous. 



LEU'TTRITE. A mineral found in 

 Leuttra, in Saxony, and thus named 

 from that circumstance. Colour, 

 grayish-white, tinged in places 

 with an ochreous brown. 



LI'AS. A provincial name, now be- 

 come conventional amongst geolo- 

 gists, for a kind of limestone, which, 

 with its associated beds, form a 

 particular group of the secondary 

 series. The name of lias has been 

 very generally adopted for a forma- 

 tion of argillaceous limestone, 

 marl, and clay, usually found in 

 comformable stratification to the 

 rocks of the oolite group. Mr. 

 Bake well considers that the name 

 lias was probably given to this for- 

 mation by the provincial pronunci- 

 ation of the word layers, as the 

 strata of lias limestone are generally 

 very regular and flat, and can easily 

 be raised in slabs from the quarry. 

 " The great bed of dark argillaceous 

 limestone, divided into thin strata, 

 called lias, is the best characterized 

 of all the secondary strata, (except 

 chalk) both by its mineral charac- 

 ters and the fossil remains imbedded 

 in it. The lias cannot be mistaken 

 for any of the lower strata; it 

 serves as a key to the geology of 

 the secondary formations in Eng- 

 land ; and the first enquiry which 

 the student should make, when he 

 is in doubt respecting the position 

 of any of the secondary beds, should 

 be, does it occur above or below the 

 lias *' 



