THE FAMILY NUMMULINID^ 233 



jjliistegina has a more advanced type of shell nearer 

 the true nummulites ; in fact, it is difficult at times 

 to be certain whether one is dealing with these forms 

 or Nummulites when its chief character, the asym- 

 metry of the test, is more or less obscure. Until 

 lately it was held by Ehizopodists that Ainpliistegina 

 had no interseptal canal system, but its presence 

 has now been proved both in the fossil and recent 

 examples. 



Operculina is typically evolute and outspread, 

 but the earlier chambers are embracing. Hetero- 

 stegina is comparable with the last-named type, but 

 the chambers are partitioned off into chamberlets. 



The discoidal test of Nummulites consists of 

 numerous chambers wound in a flat spiral, with the 

 walls of the chambers double. The intermediate 

 space is traversed by interseptal canals, which open 

 out on the surface of the test. The walls of the 

 chambers of each coil are so extended by alar pro- 

 longations or flaps that they become equitant and 

 completely cover up the preceding whorl of the 

 shell, and the test shows no signs of septation ex- 

 ternally. The sinuous lines seen on the surface of 

 Nummulites are the junctions of the alar extensions 

 of the outer septal walls of the chambers. 



The genera Cyclochjjjeus and Orhitoides, which 

 constitute the sub-family Cycloclypein.e, are examples 

 of a cyclical method of growth in these shells. In 

 the former genus there is a single layer of annular 

 chambers divided into chamberlets, and with superim- 

 posed supplementary skeleton and numerous exo- 



