426 Geological Society. 



South-eastern coast-sections of England, that the genus Micraster is 

 one and the same form gradually evolving from the more simple to 

 the more complex. In doing this, he also contends that the genus 

 may be divided into definite groups, each or several of which are 

 absolutely diagnostic of the various Chalk zones, as defined by 

 Barrois. The conclusions arrived at point to the regular and con- 

 tinuous deposition of the White Chalk, and strikingly confirm the 

 general accuracy of Barrois's zoning. 



The paper gives a minute comparison and description of the 

 genus Micraster from a general point of view, and from that of a 

 group, and deals particularly with the essential details of the test of 

 the especial groups characteristic of each zone. The Author claims 

 that, so far as Micraster is concerned, each zone is marked by a 

 definite facies of essential characters of the test, which are purely 

 horizonal, and that all species and varieties, however divergent they 

 may apparently be, occurring at any given horizon, are stamped with 

 the impress of these marked horizonal features. 



The Author proves that, while in an isolated instance, one may 

 be unable to decide the horizon in the White Chalk whence a 

 specimen of Micraster was derived, in the ninety-nine other cases 

 the diagnostic features described by him point unerringly to the 

 exact horizon, and thus afford a valuable aid to stratigraphical 

 geology, especially as the essential zonal features of the test are 

 easily made out in the field. 



The Author discusses the four groups into which the species of 

 Micraster in these zones may be placed, and describes in detail the 

 species in these groups. 



The paper is illustrated by photographs, micro-photographs, and 

 lantern-slides. 



April 12th, 1899.— W. Whitaker, B.A., F.R.S., 

 President, in the Chair. 



The following communications were read : — 



1. 'Fossils in the University Museum, Oxford: I. Silurian 

 Echinoidea and Ophiuroidea.' By Prof. W. J. Sollas, M.A., LL.D., 

 D.Sc, F.R.S. 



Attention is called to the correlation of structure and function in 

 the locomotive organs of Asterids, Ophiurids, and Echinids. In 

 the case of the two latter, movement depends on tension directed 

 along the tube-feet. In starfishes this tension is met by the dis- 

 position of the ambulacral ossicles in the form of an arch : in 

 urchins by a continuous tessellation of the surface, which would 

 only bo weakened by arch-like interruptions. If, however, urchins 

 have been evolved from an Asterid stem, they may have originally 

 possessed arch-like ambulacral grooves, and the present plates 

 of the ambulacra may have been subsequently acquired. In 

 Palwodiscus ferox of the Lower Ludlow, Leintwardine, which 

 by the structure of the buccal armature is definitely shown to 



