NUMMULINIDA I — ORBITOIDES. 



515 



Fig. 262. 



South-west of France, North-eastern India, &c.) contains a vast 

 abundance of discoidal bodies termed Orbitoides, "which are so 

 similar to Nummulites as to have been taken for them, but which 

 bear a much closer resem- 

 blance to Cycloclypeus. 

 These are only known in 

 the Fossil state, and their 

 structure can only be as- 

 certained by the examina- 

 tion of Sections thin 

 enough to be translucent. 

 "When one of these disks 

 (which vary in size, in 

 different species, from 

 that of a fourpenny-piece 

 to that of half -a- crown) 

 is rubbed-dow T n so as to 

 display its internal organi- 

 zation, two different kinds 

 of structure are usually 

 seen in it ; one being com- 

 posed of Chamberlets of 

 very definite form, quad- 

 rangular in some species, 

 circular in others, arranged 

 with a general but not 

 constant regularity in con- 

 centric circles (Figs. 262, 

 263, b, b) ; the other, less 

 transparent, being formed 

 of minuter Chamberlets 

 which have no such con- 

 stancy of form, but which 



Section of Orbitoides Fortisii, parallel to 

 the surface ; traversing at a, a the Super- 

 ficial layer, and at b, b the Median layer. 



might almost be taken for the pieces of a Dissected Map (a, a) 



Fig. 263. 





Portions of the same section, more highly magnified ; — a, Superficial 

 layer ; b, Median layer. 



L L 2 



