494 DR. A. W. ROWE ON THE GENUS MICRASTBR. [Aug. 1 899, 



32. An Analysis of the Genus Micbastee, as determined by rigid 

 zonal collecting from the Zone of Rsyncsonella Cuvieri 



to that of MiCR ASTER COR-ANGUINJJM. By Dr. A. W. EOWE, 



P.G.S. " (Read March 8th, 1899.) 



[Plates XXXV-XXXIX.] 



Contents. 



Page 



I. General Considerations 494 



II. A Review of the Evolution of the Special Features of the 

 Test in all Forms and at each Horizon, with a Tabular 



Summary 499 



III. Method of Measurement of the Test, with a Synoptical 

 Contrast between the Features of High-zonal and Low- 

 zonal Forms 514 



IV. Division of ilficrasifer into Groups 517 



(1) Group of M. cor-hovis. 



(2) Group of M. Leskei. 



(3) Passage-form from M. Leskei to M. prcecursor. 



(4) Group of M. 'prmcursor. 



(5) Sub-group of M. cor-testudinarium. 



(6) Group of M. cor-anguimim auctorum. 



(7) Sub-group of M. cor-angiiinum var. latior. 



V. Conclusions 540 



I. General Considerations. 



The Micrasters dealt with in this paper were collected from the 

 sections exposed at Gravesend, the Thanet Coast, Dover, Beachy 

 Head, the Dorset Coast, and Beer Head, in the zones indicated 

 above. 



Two thousand examples have been measured and analysed, and 

 no specimen has been included in the summary unless its zonal 

 origin is accurately determined. Six hundred photomicrographic 

 negatives of the special features of the test have been made, in 

 order that mere conjecture may play no part in the enquiry, and 

 that these important aids to specific and zonal determination may 

 be placed on a permanent and scientific footing. The field-work 

 has been extended over as wide an area as possible, so that con- 

 clusions should not be vitiated by being drawn from any one isolated 

 district. 



In this paper, the term ' low-zonal ' will apply to the zones of 

 Rhynchonella Cuvieri, Terehratulina gracilis, Holaster planus, 

 Micraster cor-testudinarium, and the lower third of the zone of 

 M. cor-anguinum (80 feet) ; and the term ' high-zonal ' will refer 

 to the remainder of the zone of M. cor-anguinum, and to the zones 

 above it.^ 



1 I hope at some future time to publish a detailed analysis of the English 

 high-zonal series, together with a notice of the sub-genus Epiaster, and of all 

 the gibbous forms. 



