^^^' 55'] ^^- ^' ^' EOWE ON- THE GENI7S MICRASTER. 497 



Many species of Micraster were described when the zonal geology 

 of the Chalk was unknown, or in its infancy, and though several of 

 these forms have since been allotted to definite horizons, it is clear 

 that their descriptions would have been of greater value had their 

 authors possessed a more extended knowledge of the genus as a whole, 

 and had they looked upon these forms rather as strong varieties, 

 occurring at certain horizons, than as definite specific entities. This 

 necessarily haphazard method of the older writers explains the naming 

 of several closelj' related forms, which are unworthy of a specific title ; 

 for had they been able to examine hundreds of zonally- collected 

 specimens, they would have been chary of making abrupt specific 

 distinctions, where gentle transition from one form to another is 

 the invariable rule. 



In reviewing this Protean genus, just as in the case of the 

 Terehratulce, it would be easy to place a series on a long table^ 

 and to show an almost imperceptible transition from one form to 

 another, and yet, from that same series, to pick out specimens which 

 would serve as distinct museum-types of the several well-known 

 species. And after all, save in the cases where the environment of the 

 urchin may have been often or abruptly changed, this is exactly 

 what one would expect. If the role of the field-worker and the study- 

 worker had been more frequently combined in the same individual,. 

 we should have had fewer unreliable species. 



Here we have a most prolific and plastic genus, prone to the 

 formation of marked varieties at any given horizon, and it is only 

 by handling a large series of examples that one can arrive at the 

 true value of these various species and varieties, and of their mutual 

 relationship in the evolution of the genus. 



One suggestive and practical point, which rigid zonal collecting 

 brings out, is that the actual conditions of existence at a given 

 horizon have at their acme so modified the evolving form that the 

 features are easily identified in the field, and become so helpful 

 as zonal guides that one can tell, with something akin to actual 

 certainty, from which zone a Micraster is derived. It is a matter 

 for congratulation that the limits of the zones, as hitherto defined, 

 coincide in a remarkable manner with the presence of well-marked 

 groupings of the characters of the Micraster-test, so that all previous 

 work in the separation of the zones not only stands good, but is 

 greatly strengthened by this further study. 



Another dominant fact, which is elicited by an enquiry based on 

 extensive zonal collecting, is that true species and strong varieties 

 cannot be, save in rare instances, picked out indiscriminately from the 

 difi'ereut horizons ; nor can they be characterized as sharply-defined 

 and separate entities, but must be regarded as mere landmarks 

 in the lite-history of the genus. To arrive at any true apprecia- 

 tion of their value, both as to the validity of their claim to specific 

 distinction and as to their usefulness as zonal guides, one must 

 examine the facies of the genus in each horizon, and then one cannot 

 fail to be impressed by the fact that passage-forms are the rule, and 

 that sharply-defined and typical species are the exception. It is 

 a.J. O.b. No. 219. 2k 



