J. IMBRIE 151 



phology. For example, taxonomic boundaries can be placed in 

 such a way that the total morphological difference between 

 corresponding parts of successive species is of the same order 

 as that between contemporary species. Application of this cri- 

 terion of course involves a subjective evaluation of total mor- 

 phological (and inferred genetical) differences. 



The Micraster classification now in use and illustrated in Fig. 

 15 is based on a combination of the three criteria discussed 

 above, with emphasis on the morphology. Note that the inter- 

 pretation of the M. leskei-corangiiinum lineage as the main line 

 of Micraster evolution is reflected in the failure to subdivide 

 M. leskei and M. cortestudinarium. 



Summary 



A review of recent literature in paleontology indicates that 

 the nature of fossil species remains a controversial matter, al- 

 though the biological species concept based on variable, inter- 

 breeding populations has largely replaced the typological con- 

 cept based on morphotypes. Data on morphology, association, 

 biogeography, paleoecology, and biostratigraphy are used by the 

 paleontologist to delineate fossil species. Most species actually 

 defined by neontologists and paleontologists are transient species, 

 i.e., essentially contemporaneous aggregates of interbreeding pop- 

 ulations. In rare instances paleontologists describe segments of 

 phyletic lineages. Groupings of this sort are called successional 

 species. 



The principal source of difficulty in applying the biological 

 species concept to fossils is the incompleteness of the available 

 fossil record. This lack is attributed to a combination of factors, 

 including nondeposition, erosion, migration, nonpreservation, and 

 inaccessibility. Taxonomic problems also arise from the limita- 

 tions inherent in morphological data and the prevalence of biased 

 frequency distributions. In dealing with successional species the 

 problem arises as to the selection of criteria for subdividing con- 

 tinuous lineages. Taxonomic boundaries may be designated on 

 the basis of phyletic branching, stratigraphy, or morphology. 



