T. Crook— On Dedohmitimtion, 339 



the terminal plate, so that the other plates of the body are, as in the 

 Girvan form, emarginate in front instead of behind as he described. 



H. Gray ice, (Woodward) * is a much more slender and elongated 

 form, and the coarse ornamentation of the plates is quite different, 

 but in some specimens recently found by Mrs. Gray there are in- 

 distinct indications that the shell was minutely perforate as in 

 Hel. thraivensis. 



EXPLANATION OF PLATE XV. 

 Fig. 



1. Helminthochiton thraivensis, sp. nov. Side view of internal cast of 



complete individual, showing the eight plates in position. Nat. size. 

 h = head-plate. 

 la. Ditto. Head-plate of same specimen, x 2. 



2. Ditto. Internal cast of the five anterior plates of another specimen. 



Nat. size. 

 2a. Ditto. Second plate (h 2) of same specimen, x 2. 



3. Ditto. Side view of internal cast of the two posterior plates of another 



specimen. Nat. size. t = terminal plate. 

 3a. Ditto. Transverse vertical section of same specimen, showing axial bending 

 of plates. Nat. size. 



4. Ditto. External impression of three successive plates, showing anterior 

 emargination. x 2. 



External impression of another plate, showing general shape, x 2. 

 Portion of surface of adjoining plate, showing large and small 



x6. 

 Portion of ditto near margin, showing concentric arrangement, x 6. 

 External impression of two other plates, x 2. 



Natural longitudinal vertical section of complete individual, showing 

 mass of spicules. Nat. size. h = head-plate. 

 la. Ditto. Side view of internal cast of three successive plates of same 



specimen. Nat. size. 

 lb. Ditto. Internal cast of head-plate of same specimen, viewed from 



above, x 3. 

 7c. Ditto. Portion of spicular mass of same specimen. x3. 

 8. Ditto. Diagrammatic restoration of complete individual. 



N.B. All the specimens come from the Starfish Bed (Drummuck 

 Group), Thraive Glen, Girvan, and are in Mrs. Gray's Collection. 



II. — On Dedolomitization. 



By T. CROOK, A.E.C.Sc. (Dublin), F.G.S., Scientific and Technical Department, 

 Imperial Institute. 



IT is a noteworthy feature of the most ancient crystalline limestones 

 that they are generally dolomitic, or at any rate frequently so. 

 They often consist of practically pure dolomite ; whilst associated 

 with this in some cases is pure crystalline magnesite, 2 which is very 

 liable to be mistaken for saccharoidal dolomite. t 



Crystalline dolomites, dolomitic limestones, and their dedolomitized 

 equivalents have a world-wide distribution among the usually much- 

 metamorphosed pre-Cambrian rocks. In North America they are well 



1 Woodward, Geol. Mag., Dec. Ill, Vol. II, p. 352, PL IX, Figs. 7-10, 1885 ; 

 Heed, ibid., Dec. V, Vol. IV, p. 113, PI. IV, Fig. 12, 1907. 



2 Such crystalline magnesite occurs in Ceylon, as at Kandeniya, Wellawaya. 

 It occurs also in various parts of North America (see Dana's System of 

 Mineralogy, 6th ed., p. 275) and elsewhere. 



