A. J. Jukes-Browne — T/ie Vale of Marshivood. 161 



in one of them the ambulacral furrow is considerably larger than in 

 the other. It seems unnecessary to regard this as a specific or 

 varietal diiference, for it may be either sexual or a seasonal variation, 

 the anterior furrow being enlarged to serve as a marsupium. 



EXPLANATION OF PLATE V. 



Figs. \a and \b. Hhahdooidaris Libyensis, nov. sp., from the side and from above ; 



nat. size. Fig. Ic, ambital interambulacral plates of the same, x 2 diam. 



Fig. Id, ambital ambiilacral plates of the same, x 4 diam. 

 Fig. 2. Coptosoma Thevestense, Per. & G., ambital plates, x 4 diam. Turonian: 



Abu Eoasch. 

 Fig. 3. Psaw»?ecAM??<s 7)Mci«, "Wright, ambital plates of Egyptian specimen, x 5 diam. 

 Figs. 4 and 5. Psammechinus Lyonsi, nov. sp. Fig. 4a, side view, x 2 diam. ; 



Fig. 4i, actinal view, x 2 diam.-; Fig. 4c, ambital plates, x 8 diam. Fig. 6. 



Ambital interambulacral plates of another specimen, x 8 diam. 



EXPLANATION OF PLATE VI. 



Figs. \a and \h. Echinolampas tumidopetalum, nov. sp., abactinal and lateral views ; 

 nat. size. 



Fig. 2. Conoclypeus Delanoiiei, Lor., var. milviformis, nov., actinal surface; two- 

 thirds nat. size. 



Fig. 3. Linthia Esnehensis — Aschersoni, apical disc, X 5 diam. 



Fig. 4. Sypsopatagus, sp., side view; nat. size. 



III. — The Origin of the Vale of Marshwood in West Dorset.^ 



By A. J. Jukes-Browne, B.A., F.G.S. 



rpHE great sheet of Chalk which, with the subjacent Greensand 

 jL and Gault, stretches through so large a part of Southern 

 England arwl underlies the whole of the Hampshire Basin, termi- 

 nates abruptly in West Dorset. There is no doubt that the Upper 

 Cretaceous rocks once spread continuously over the Jurassic hills 

 east of Bridport and across the Vale of Marshwood, and were united 

 to the corresponding beds in East Devon, where the Chalk and 

 Greensand are so conspicuous in the cliffs near Beer Head. 



To those who ai'e unacquainted with geological methods this 

 statement may seem highly imaginative, since, at the present time, 

 there is a broad intervening tract, from the centre of which all 

 traces of Cretaceous strata have been removed, and around which 

 onl}'^ a few isolated patches or outliers of Greensand remain as relics 

 of their former extension ; yet to the eye of a geologist these very 

 outliers, of which Pilsdon Pen is one, are clear and certain proofs 

 that a continuous sheet of the same material once overspread the 

 whole area. 



^'he physical features of this area may be brieflj'- described, as they 

 are not likely to be familiar to those who do not live in the west of 

 Dorset. The Vale of Marshwood is an area of low ground, most 

 of which lies between 100 and 200 feet above the sea; its length is 

 about five miles and its breadth three ; its floor consists of tlie 

 clays of the Lower and Middle Lias, and it is encircled by steep 



^ This paper is reprinted, with some alterations, from the Proceedings of the 

 Dorset Nat. Hist, and Ant. Field Club, vol. xviii, and is published with the 

 permission of the Director- General of the Geological Survey of Great Britain. 



DECADE IV. VOL. T. NO. IV. 11 



