262 E. B. Bailey — Drakes Island, Plymouth. 



here called the left anterior. This fact does not necessarily upset the 

 hypothesis of a primitive tri-radiate form, but it suggests further 

 research. 



EXPLANATION OF PLATE VI. 

 Adoral Surfaces of Megacystis. 

 The drawings with numbers underneath are from specimens in the British 

 Museum, and are of the natural size. Those provided with names are copied 

 from Miller's original figures, and are presumed to be of the natural size. In 

 all the drawings the details of ornament have been omitted, so as to emphasize 

 the suture-lines and thecal openings. 



FlG. 22. — E 7630. Hydropore distinct. Gonopore apparently with two 

 openings near left post, facet. 

 ,, 23. — M. commodus, after S. A. Miller, 1891, Advance Sheets 17 Eep. 

 Geol. Surv. Indiana, pi. iii, fig. 2. The suture between the 

 posterior Ad. I, here dotted in, is not shown in the original 

 figure. 

 ,, 24. — E 7631. Hydropore indicated by a faint dotted line. Gonopore on 



post. Ad. II. 

 ,, 25. — M. omatissimus, after S. A. Miller, 1891, op. cit., pi. v, fig. 2. 

 ,, 26. — E 7641. Gonopore on post. Ad. II. Periproct crushed. Holes 

 due to boring organism seen in N.W. quarter and a small 

 pelmatozoan root on left interradial Ad. II. 

 ,, 27.— If. splicer oidalis, after Miller & Gurley, 1895, Bull. Illinois State 



Mus., vii, pi. v, fig. 3. Ad. I not shown in the figure. 

 ,, 28.— M. splendens, after Miller & Gurley, 1894, op. cit., v, pi. i, fig. 9. 



Suture between post. Ad. II not shown. 

 ,, 29. — E 7673. The sutures between the posterior adorals are obscure. 

 ,, 30. — M. gyrinus, after Miller & Gurley, 1894, torn, cit., pi. i, fig. 3. 

 Hydropore low down on suture between post. Ad. I. Gonopore 

 near base of left post, facet. 



1 



IV. — Drake's Island, Plymouth. 

 By E. B. Bailey, M.C., B.A., F.G.S. 



MLE main feature of the geology of Drake's Island is the association 

 of volcanic rocks with marine limestone of Mid-Devonian 

 age. The Geological Survey memoir 1 by Mr. "W. A.. E. Dssher 

 attributes the discovery of the volcanic rocks to Mr. E. N\ "Worth. 

 Mr. TJssher leaves their precise nature, "whether lavas or tuffs," 

 somewhat in doubt, but states that they are "most certainly 

 contemporaneous". As a matter of fact both lava and tuff occur, 

 and are definitely distinguishable. 



The limestones A and C of the sketch-map are attributed by 

 Mr. TJssher to the Plymouth horizon, and the double outcrop is 

 interpreted as a repetition due to plication. It is certainly possible 

 that there is only one limestone — reduplicated by faulting — but 

 I think it more probable that the limestone C is distinct from A," 

 and is interstratified between the tuffs P» and the lava D. At any 

 rate the tuffs P> are not very like the tuffs E, for they are much less 

 distinctly bedded, and they much more commonly contain fragments 

 of limestone. 



On broadly based grounds of succession and structure, Mr. Ussher 

 regards the Plymouth Limestone as older than the Drake's Island 

 1 The Geology of the Country around Plymouth and Lisheard, 1907. 



