546 DE. A. W. EOWE ON THE GENUS MICKASTEE. [Aug. 1 899, 



Fig. 5. Sub-anal fascicle of Micraster prcBcursor of medium size, Holaster 

 jjlanus-zone, Dover. x3. 



6, Sub-anal fascicle of M. prcecursor of medium size, M. cor-testudinarium- 



zone, Seaford Head. X3. 



7. Sub-anal fascicle of M. cor-anguinum of medium size, M. cor-anguinum- 



zone, GraTCsend. All three specimens were picked as being of the 

 same size, so that the contrast might be legitimate. X 3. 



Discussion. 



Mr. W. Hill congratulated the Author on the excellence of the 

 paper and the admirable illustrations. He hoped that he could 

 congratulate Cretaceous geologists also on the removal of a difficulty 

 which had troubled them for many years. He recognized in the 

 upper part of the Middle Chalk Micraster cor-bovis^ and in the base 

 of the Upper Chalk M. cor-anguinum. In the Chalk between 

 the horizons at which these species occurred were a number of 

 foims, each differing slightly from the other, some of which he 

 referred to M. brevijporus and others to M . cor-testudwarium, but he 

 had never been satisiied with their identification, and for many years 

 it had been felt by himself and others that they were not treading on 

 firm ground. He did not propose to criticize the characters by which 

 the Author distinguished the four species, or the new form M. pra- 

 cursor. The Author had kindly explained to him the guiding 

 characteristics of the Micrasters, and he had followed these in the 

 field. He could endorse all that the Author had said with regard 

 to the gradual change of the ambulacral areas, the tuberculation of 

 the test, the character of the mouth, and its position in regard to 

 the ambitus ; but he was not yet certain as to the stratigraphical 

 value of these characters. This point, however, would perhaps 

 become clearer when they had the paper before them, and could test 

 ^ it in the field. 



Prof. Seeley remarked that the Author's diagrammatic arrange- 

 ment of his specimens in parallel series corresponding to the zones 

 from which they were collected, clearly exhibited the vertical 

 survival of the gradated forms termed 'species'; and the photo- 

 graphic enlargements of the poriferous zones and other parts 

 of the test had demonstrated the typical characters by which 

 the species might be distinguished. There were variations seen 

 in every chalk-pit ; and he desired to ask the Author whether 

 there was, in the vertical series of specimens examined, any 

 evidence of variation in the distinctive characters of species which 

 would amount to evolution within the genus Micraster. It was 

 most desirable to correlate the vertical variation with horizontal 

 geographical variation, and to learn whether the horizontal variation 

 had any relation to the vertical variation which might challenge the 

 identification of horizons in the Chalk by the varietal terms. He 

 desired to know which of the specific characters become efifaced, and 

 where the obliteration has been observed. 



Dr. J. W. Gkegoet thought this paper of great interest, as 

 it applied to the question of the systematic value of the differences 

 in the Chalk Micrasters the only method that is likely to yield final 



