6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 89 



The zonation of the faunas in the foregoing lists is imperfect. The 

 old problem of where the boundary between the Oligocene and the 

 Miocene should be placed in the American Tertiary sequence still re- 

 mains unsolved, and zonal studies of the faunas have not yet been 

 made with sufficient refinement in the Tertiary formations of Mexico, 

 Central America, and the West Indies. Whether zones can be recog- 

 nized in the Mexican Meson and San Rafael is not established, but 

 it is probable that they can be. There may be several horizons in the 

 Cuban Oligocene, and zones may later be recognized in the Antigua 

 formation of Antigua. The zoning of the formations in the Panama 

 Canal Zone is not yet satisfactory. To undertake a detailed discussion 

 of these stratigraphic perplexities, although they are of great im- 

 portance, is not quite germane to the present paper, but attention 

 should be directed to the facts above stated. 



VARIATION IN SPECIES OF LEPIDOCYCLINA 



Variation in several species of Lepidocyclina is discussed in some 

 detail on subsequent pages. They are L. waylandvaughani, L. parvula, 

 L. yurnagunensis, and L. favosa. The amount of variation in many 

 species of orbitoids is bewildering. Because of such variation and 

 the difficulty of defining certain species, I have for years delayed 

 publication on some of them. It would be expecting too much to hope 

 that all interpretations made in this paper will remain unchallenged 

 or unchanged. A more reasonable hope is that this study may help 

 in a very difficult kind of research. 



Normally, in orbitoids there are two kinds of dififerences that are 

 not due to variation, as follows : 



First, the difference due to the alternation of microspheric and 

 megalospheric generations. For each species, before its definition can 

 be completed, the two forms need to be found. For some species 

 there is uncertainty regarding the proper association of the two forms. 

 An example is the Lepidocyclina gigas — L. undosa couple. The former 

 is probably the microspheric and the latter the megalospheric form 

 of the same species, but as the suggestion of such a relation is still only 

 tentative, I am not calling one form B and the other form A of one 

 species. 



Second, the difference due to relative age. Orbitoids grow by the 

 extension of the equatorial layer and by the addition of lateral 

 chambers one above another. Not all features are fully developed 

 in young specimens. There may be marked differences in the ratio, 



lame er ^ j^ young and old specimens, and pillars, although well de- 

 thickness 

 veloped in old, may be absent in young specimens. Differences of the 



