4 o 4 SCIENCE PROGRESS 



fairly constant. In the Tilmanstone pit these two lower zones 

 combined have a thickness of about 40 ft. : at Folkestone they 

 are 35 ft. thick. 



It is exceptional, rather than the rule, that a species of fossil 

 should have a sharply defined and restricted distribution. As 

 examples we may quote Uintacrinus, a free-swimming crinoid, 

 whose unmistakable, deeply-grooved, irregularly pentagonal 

 plates are very abundant in the basal portion of the zone ol 

 Marsupites. The belt in which this species occurs is only 62 ft. 

 thick in East Kent, and it is absolutely restricted to this belt : 

 consequently the discovery of one of these plates in a quarry 

 or a pit is certain proof of the presence of that particular 

 horizon. 



Another striking instance is afforded by Actinocamax plenus. 

 This is a belemnite which occurs commonly in a thin bed of 

 marl which caps the Cenomanian or Lower Chalk. The 

 discovery of this belemnite and its marl seam, only a few feet 

 thick, is an absolutely undeniable proof of the junction of the 

 Middle and Lower Chalk. 



Further very valuable evidence is afforded by the genus 

 Micraster, a kind of sea-urchin which is abundantly represented 

 in the lower zones of the Senonian or Upper Chalk. Rowe has 

 shown that in the forms from the lower horizons the ornamen- 

 tation and sculpture of the test are simple, but that it becomes 

 more and more ornate as we trace the evolution upwards. He 

 has worked out in minute details the evolution of the test in 

 an unbroken succession of gradations with such accuracy that 

 it is possible to determine the horizon by the examination of 

 the Micrasters alone, and this fact has been of very great value 

 in delimiting the zones passed through in the sinkings. Two 

 species of this genus, namely Micraster cor-testudinarium and 

 Micrastcr cor-angiiimtm, are chosen as indices, or species which 

 give their names to zones : these species are not rigidly 

 confined to the zones in question, but are specially abundant 

 in them. 



For zonal work, whether in the field or in the pits, it is not 

 usually enough to find single specimens, but all available 

 material should be searched in order to present a collective 

 idea of the fauna as a whole. 



In the sinking of the shafts at the three collieries, the fossils 



