Vol. 51.] RADIOLARIAN ROCKS IN THE LOWER CULM MEASURES. 665 



The beds are mainly of limestone and shales in the lower portion 

 of the quarry ; above these, beds of cherty rock several feet in thick- 

 ness are interbedded with limestone. A microscopic section of the 

 chert showed radiolarian casts in places. According to the late 

 Mr. Gr. W. Ormerod, 1 Posidonomya occurs in one of the lower beds 

 of limestone in this quarry. 



Judging by the above-mentioned sections, there seems good reason 

 to suppose that the Radiolarian chert is present along the northern 

 margin of Dartmoor in the same relative position above the Posido- 

 nomya-MmestonQS, and shales as in the other districts described in tbe 

 present paper. More particularly there is a very close resemblance, 

 both in the character of the chert and its relation to the associated 

 limestones, to the beds exposed at the Wooladon quarry near 

 Launceston (see p. 623).— July 22nd, 1895.] 



EXPLANATION OF PLATES XXIII.-XXVIII. 

 Plate XXIII. 



Greological Sketch-map of Devon, West Somerset, and Cornwall, on the 

 scale of 10 miles to the inch. 



Plate XXIV. 



Section of the Radiolarian (Codden Hill) Beds of the Lower Culm Measures 

 at Carzantic Quarry, near Launceston. From a photograph taken in 

 March, 1895. 



Plate XXV. 



The specimens figured in the following three Plates are all from the Radiolarian 

 Beds of the Lower Culm Measures. 



Figs. 1 a-\ g. Sphcerozoum, sp. Detached spicules of various forms. From 

 the Lower Culm Radiolarian rocks at Swimbridge, Heddon, and 

 Kent's Hill, North Devon ; Carzantic, near Launceston ; Ramsho.ru 

 Down ; Winstow Lodge, near Chudleigh. X 200 diameters. 



Fig. 2. Cenosphcera pachy derma, Rust. Ramshorn Down, near Bovey Tracey, 

 South-east Devon. X 200. 



3. Cenosphcera, sp. a. From near Ashbrittle, Wellington, West Somerset. 



X200. 



4. Carposphcera, sp. a. Ramshorn Down, South quarry. X 200. 



5. Carposphcera, sp. b. Artiscombe quarry, near Tavistock. x200. 



6. Carposphcera, sp. c. From a boulder of radiolarian chert at Tavistock. 



X200. 



7. Carposphcera, sp. d. Ramshorn Down, South quarry. X200. 



8. Thecosphcera, sp. a. From hedge-section, Lawell House, Chudleigh. 



X200. 



9. Xiphosphcera, sp. a. Ramshorn Down, North quarry. X200. 



10. Xiphosphcera, sp. b. Ramshorn Down, North quarry. X200. 



11. Xiphostylus, sp. a. Ramshorn Down. X200. 



12. Xiphostylus, sp. b. Showing traces of the lattice-structure, Ramshorn 



Down. X 200. 



13. Xiphostylus?, sp. c. Ramshorn Down. X200. 



14. Statirolonche?, sp. a. Hedge-section, Lawell House, near Chudleigh. 



X200. 



15. Stylosphcera, sp. a. Ramshorn Down. x200. 



16. Cenellipsis, sp. d. Ramshorn Down. X200. 



17. Druppula, sp. a. Ramshorn Down. X 200. 



1 Trans. Devon. Assoc, vol. ii. (1867-68) p. 124. 



