12 The Marsupites Chalk of Brighton. 



to suppose tbat the adoral region has been subjected to any such 

 crushing or pulling as might have dragged it away from the rest. 

 The turret then, for purposes of this description, is limited to the 

 smaller plates, although in origin they must be of the same nature 

 as the cup-plates. 



The turret is clearly preserved for a distance of 13 mm. below the 

 cup ; beyond that distance, for 7 mm., tbe plates are much disturbed, 

 but it is quite probable that the total height of the turret was 20 mm. 



The diameter of the turret at its upper (proximal) end is 4'3 mm. 

 Thence it tapers gradually, till at 13 mm. from the cup the diameter 

 is 3*2 mm. Thus the mean diameter is about one-fifth of the height. 



The turret is slightly curved. 



The turret-plates imbricate adorally, and form a series of tiers. 

 In the greater part of the turret the plates in one tier alternate quite 

 regularly with those of the adjacent tiers. Thus the plates come 

 to lie in definite vertical series or columns, those of one series 

 alternating with those of the adjacent series. As the stem tapers 

 distally the vertical series come closer together and the regular 

 arrangement is lost. The distance between the tiers seems to 

 become rather less as the proximal (upper) end of the turret is 

 approached. In the upper half of the turret, i.e. in the proximal 

 10 mm., there are about 22 plates in vertical series, so that the 

 usual amount of each plate exposed is about '5 mm., a little less 

 in the immediately proximal region, but scarcely ever more in the 

 distal region. The number of tiers in the same distance is double, 

 viz. 44 ; since there would be fewer in the distal region the 

 total may be estimated at about 80. The number of vertical series 

 visible in the proximal region is 5, from which a total of 10 at most 

 may be inferred. 



In spite of the regularity of tiers and columns, the wicker pattern 

 produced is not very regular, and this is due to the ragged outline 

 of the individual plates. Probably the adoral margin of each 

 plate tended to form a convex curve ; but the edge was thin and 

 irregular, and has frequently been broken, so that the marginal 

 curve is truncated. 



It is quite possible that the free edge of the turret-plates 

 supported minute spinules, but there is now no trace of such 

 structures. At the very summit of the turret, however, lying 

 against the outer cup-plates, are four vertical rod-like ridges; and 

 these may represent spines. 



(To be concluded in our next number.) 



III. — The Marsupites Chalk of Brighton. 

 By B. M. Brydone, F.G.S. 



A DESCRIPTION on modern lines of the Brighton chalk below 

 the (old) zone of Aetinocamax quadratus is to be found in 

 Dr. Rowe's "Coast Sections", pt. i, p. 346. ' The principal points 

 that he makes are that between Ovingdean Pumping Station and 

 Black Bock the cliffs display below the (old) zone of A. quadratus 

 1 Proc. Geol. Assoc, vol. xvi, pt. \i. 



