5i0 DK. A. W. EOWE ON THE GENUS MICRA.8TEK. [Aug. 1 899, 



that it is merely the broad variety of Micraster cor-anguinum 

 auctorum, just as M. cor-testudinarium is but the broad variety of 

 M. prcecursor. 



These broad forms have no special features of the test which are 

 not shared by the narrow type. The paired ambulacra are shown 

 in PI. XXXVI, figs. 7 & 8 ; the single ambulacrum in PI. XXXYII, 

 fig. 5 ; and the features of the base in PL XXXVII, fig. 9. 



It is needless to give a special figure of this variety, because, save 

 only in breadth, it is the same in profile-variations as M. cor- 

 anguinum, PI. XXXY, lines vi & vii. 



Length. Breadth. Height. 



p.. . / Large size 68mm. 68mm. 47 mm. 



•^^°^®^^^°"^ 1 Medium size ... 47mm. 50mm. 30mm. 



Y. Conclusions. 



The object of this paper is to show that from the beginning of the 

 low-zonal series to the beginning of the high-zonal series — from the 

 zone of Bhynchonella Cuvieri to the upper part of the zone of M. cor- 

 anguinum — we can trace an unbroken continuity in the evolution of 

 Micraster ; so that as we mount up, zone by zone, fresh features are 

 added to the test, simply owing to the progressive elaboration of 

 the epistroma ; and that in each zone the special features of the 

 test are so marked that one can tell by their aid from what zone 

 a Micraster is derived. 



In this particular connexion it is necessary to say that, with 

 the exception of a few details of measurement, the work of 

 Dr. Charles Barrels, on the Chalk of the South and South-east 

 coasts of England, stands this severe test of zoological evolution, 

 and it is a privilege to record here the value of his zones, and the 

 delight which it has been to verify the general conclusions arrived at 

 in the invaluable monograph contributed by him to British Geology. 

 A paper is already in hand which will give in detail a description of 

 the zones of the Chalk, as seen in the cliff-sections of Kent, Sussex, 

 Dorset, and Devon, in order that in future zonal collecting may be 

 more easily accomplished. 



It has been thought well to separate the genus into two main 

 zoological divisions — fhe high-zonal and the low-zonal ; and it is 

 found that the zoological break takes place at about the top of the 

 lower third of the zone of M. cor-anguinum. There are, however, 

 two great points of departure in the life-history of the genus — one, 

 just mentioned, where the low-zonal features begin to be replaced 

 by the high-zonal ; and the other in the Holaster jdanus-zone, where 

 the thin-tested M. cor-hovis passes into the thick-tested M. LesJcei, 

 which again merges into the passage-form between it and M. prce- 

 cursor, the latter breaking up into typical M. prcecursor and M. cor- 

 testudinarium. 



The writer has felt impelled to take up a firm position on the 

 species question, for if ever an extinct genus gave scope for the 

 endless multiplication of specific types, that genus is Micraster. 



