544 DK. A. W. ROWE ON THE GENUS MICRASTER. [Aug. 1 899, 



and the anterior segment of the peristome. The chief difficulty, 

 however, has been experienced with the ambulacra, for it seems 

 well-nigh impossible to obtain at the same time the detail of the 

 ornamentation and the suggestion of the depth of the ambulacral 

 groove. I have had, therefore, to abandon the latter in favour of the 

 former, which is all-important, and to be content to demonstrate the 

 latter with horizontal sections, as shown in PI. XXXYI. Whatever 

 their imperfections as photomicrographs may be, they give a more 

 faithful representation of the features of the test than any drawing 

 could give. Moreover, this process has one great advantage to re- 

 commend it — that I have been able to take dozens of negatives of 

 each special feature, and that from scores of specimens from each 

 zone, and by this means I have obtained a broader and more truly 

 comparative idea of such features thau could be secured by merely 

 glancing at the specimens with a leus. Such is the advantage of 

 the permanent photographic record over the fleeting mental im- 

 pression. The cost of obtaining equally numerous figures through 

 the medium of a draughtsman would be prohibitive. 



The results embodied in this paper have been arrived at under 

 difficulties, as they have absorbed most of the small leisure of a busy 

 professional life during the last four years. They are advanced, 

 after careful study, in the hope that they may prove of value both 

 to the amateur and to the professional geologist. 



For the last three years Mr. C. Davies Sherborn has accompanied 

 the writer in the field and, both there, and in the study, has seen the 

 growth of the writer's ideas, and has helped to verify his statements. 

 To him, before all, the writer desires to express his gratitude for 

 friendly and stimulating interest. 



Dr. Barrels, Mr. E. T. Newton, Mr. W. Hill, Mr. Jukes-Browne, 

 and Mr. Henry Woods have gone over the results in the most 

 patient manner, and have seen most of the specimens, and thanks 

 are due to them for their kindly interest and suggestions. To 

 M. Jules Lambert, M. V. Gauthier, Dr. J. W. Gregory, Mr. C. Griffith, 

 Mr. Meyer, Mr. G. E. Dibley, and others, the writer is indebted for 

 much assistance in the matter of specimens, literature, and strati- 

 graphical information, which have greatly helped the enquiry ; and 

 on this head he would give cordial recognition to the generosity 

 which has prompted Major-General Cockburn to place his large and 

 beautiful collection of Dover Micrasters at the writer's disposal. 



To all these friends the writer is very grateful, and his sincere 

 hope is that this paper will be found of use to those whose friendly 

 interest has been of so much service to him. 



EXPLANATION OF PLATES XXXV-XXXIX. 



Plate XXXV. 



This plate is intended to demonstrate that certain dominant profile- shapes 

 persist throughout the zones, in all species contained in the zones mentioned, 

 whether they be broad or narrow forms. For full explanation, see p. 509, 



