136 Beporfs and Proceedings — 



and Cystohlastus some or all of the interradials enter the radial ring. 

 Glyptocystis affords a transition to such flattened forms as Pleurocystis. 

 Homocyntis, lichenoides, Macrocystella and some other types have 

 three regularly alternating series of plates. 



Orals, homologous with those of Crinoidea, are to be distinguished: 

 (a) five of equal size, e.g. Cyathocystis of Eussia ; (/3) five of unequal 

 size, the posterior being the larger, e.g. Glyptosphcsra Leuchtenhergi 

 and Sphm-onis ; (ty) none in adult, probably resorbed, e.g. Ascocystis, 

 Pirocystis, Cryptocrimis. 



The apertures on the summit are as follows : — 1. Central=Mouth ; 

 2. Large lateral = Anus ; 3. In the anal interradius between mouth 

 and anus, sometimes close to the anus, sometimes even in the same 

 genus (e.g. Aristocystis), further from it, = Genital aperture ; in some 

 cases this is contained in the anal aperture as in the recent starfish 

 Hymenaster and PyOionasier ; 4. A small opening by the side of the 

 genital = Excretory aperture ; this is seen in Aristocystis, and, as a 

 " madreporic plate," in Glyptosphcera. In Echinoencrinus it is not so 

 much in the anal interradius (C D) as in the next one (D E), and 

 the single opening occupying this position in Cystohlastus, Glyptocystis, 

 and other tj'pes, is probably of the same nature. 



Geological Society op London. 



L— January 21, 1891.— A. Geikie, Esq., LL.D., F.E.S., President, 

 in the Chair. — The following communications were read : — 



1. " On the Age, Formation, and Successive Drift Stages of the 

 Valley of the Darent ; with Kemarks on the Palajolithio Implements 

 of the District, and on the Origin of the Chalk Escarpment." By 

 Professor Joseph Prestwich, D.C.L., F.R.S., F.G.S., etc. 



i. General Character and Age of the Darent Valley. — The river is 

 foi'med by the union of two streams, the main one flowing east from 

 near Limpsfield, the other west from near Ightham, pai-allel with the 

 ranges of Lower Greensand and Chalk, and flows northward into the 

 Thames. The first indent of the valley was subsequent to the depo- 

 sition of the Lenham Sands, and indeed to the Eed Clay with flints 

 and the old implement-bearing drift with which this is associated ; 

 and the same remark applies to a system of smaller valleys starting 

 near the crest of the escarpment and running into the Thames. 



ii. The Challc Plateau Drifts and associated Flint Implements. — 

 Since the publication of the author's Ightham paper, Mr. Harrison 

 and Mr. De B. Crawshay have found implements mostly of rude 

 type (though a few are as well finished as those of Abbeville) in 

 numerous localities on the plateau, where, owing to the gradients, 

 the difference of level between plateau and valley-bottom is much 

 greater than at Currie Farm. Evidence derived from the character 

 and conditions of preservation of these implements is adduced in 

 favour of their great antiquity. 



iii. The Initial Stages of the Darent Valley. — The author has pre- 

 viously shown that in early Pliocene times a plain of marine 

 denudation extended over the present Vale of Holmesdale, and that 

 in pre-glacial times the plain was scored by streams flowing from 

 the high central Wealden ranges. These streams centred in the 



