ON THE FAMILY OF RUDISTA. 



133 



Deshayes stated, but are, as M. Goldfuss observes, members 

 of the class Brachiopoda. From a minute examination of 

 the characters, the class Brachiopoda may be divided into two 

 orders, B. reguliers and B, irreguliers, or Rudista ; the former 

 including those with loose shells, the latter those with fixed 

 shells. The second order is divisible into two families, 

 Hippurites and Caprinidce ; in the first of which he places 

 the genera Hippurites, Radiolites, and Crania, and in the 

 second the genera Caprina and Caprotina. 



It is worthy of remark, as one of the prominent Geological 

 considerations, that the Rudistidae are not disposed generally 

 throughout all formations, (r. Geologist, p- ^2), but are dis- 

 posed in bands, forming geological horizons, which are so 

 much the more remarkable, as they may be traced through a 

 large extent of Europe ; and that these bands, as in the dis- 

 trict of Corbieres, have suffered no disturbance, but that the 

 Rudista are in the same situations as those in which they 

 existed previous to their destruction. 



The Rudista first appear in the strata of the neocomien 

 period ; this band, to which M. Elie de Beaumont long since 

 called attention, under the title of " calcaire a dicerates,'' 

 forms a thick zone, which may be traced through the basin 

 of the Mediterranean, from Martigues to Chambery, and in- 

 cludes four distinct species. Leaving the boundary of this 

 zone, not a single specimen is to be found throughout the 

 Gault, whilst, in the chlorite-chalk, of the south-west of 

 France, another band of Rudista is found, forming the second 

 zone of M. d^Orhigny ; this contains fourteeu species, all of 

 which differ from those occurring in the first zone. After 

 ascending in the series, and passing several very extensive 

 beds, a third zone of Rudista is found, which extends through- 

 out the basins of the Pyrenees, and the Mediterranean, and 

 may be traced into Egypt, through Moravia, Bosnia, and 



