R. M. Brydone — Zone of 0. jyilula, S. English Chalk. 359 



Fig. 3. Trinucleus concentricus (Eaton), x 2. Middle Bala : Onnj Eiver, 

 Shropshire. 

 ,, 3a. Ditto. X 2. Side view of same specimen. 



,, 4. Trinucleus favus,'&^\iex. x 3. Side view. LlandeiloBeds : Llandeilo. 

 ,, 5. Trinucleus gibbifrons, McCoy, x 2J. Middle Bala : Wales. One 



of the specimens named and labelled by McCoy. 

 ,, 5a. Ditto. X 2^. Side view of same specimen. 

 ,, 6. Ditto. X 2^. Middle Bala: Gelli Grin. 



,, 7. Ditto? X 2i. Starfish Bed, Thraive Glen, Girvan. Mrs. Gray's 

 Collection. 

 N.B. All the specimens, except mentioned otherwise, are in the 

 Sedgwick Museum, Cambridge. 



{To be continued.) 



V. — The Zone of Offaster pilula in thk South English Chalk. 



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



Part I. 



IN the year 1911 Mr. Griffith and I ' proposed to recognize in Hants 

 three divisions of the Chalk intervening between the zones of 

 Marsupites testudinarius and Belemnitella mucronata, and then known 

 as the zone of Actinocamax quadratus. Tliese divisions we were not 

 jointly prepared to treat as more than subzonal. In the year 1912 

 I^ defined the lower and middle divisions very closely and described 

 them in detail, and demonstrated that the upper boundary of the 

 middle division was a strongly marked break. This break I regarded 

 as of zonal importance, and I therefore separated the lower and 

 middle divisions from the upper as subzonal divisions of a new zone 

 of Offaster pilula. I was also able to say that the features of the new 

 zone in Hants were exactly reproduced in the Sussex cliffs and to 

 cite as presumptive evidence that it was equally recognizable in 

 Wiltshire a statement by Dr. Blackmore that the divisions proposed 

 in 1911 held good round Salisbury. The object of this paper is to 

 trace the new zone in detail in the moi'e accessible cliff sections of 

 the old zone of A. quadratus in the South of England. 



The zone of 0. pilula, as established for Hants (on inland sections 

 only), was estimated to be about 105 feet thick, made up as follows : — 



Feet 



Si 



/ 



Upper belt of 0. inlula, with a pair of marl seams 

 about 18 inches apart as its base and a marl seam 

 ° ?^ (succeeding about 3 feet of chalk yielding 0. pilula 



g O J of exceptional size) as its top .... about 13 



_N g I ' Middle belt,' with few 0. pilula and apparently no 



marl seams ....... about 20 



Lower belt of 0. pilula, bounded above and below by 

 marl seams ....... about 12 



„ ^- 2 ^Chalk with O.^jiZwZa occurring erratically and becoming 

 ° -S ^ I very scarce towards the bottom ; Echinocorysscutatus, 

 c -2 v^ I var. depressus, in the upper part ; E. scutatus, 

 N o ^ "I var. tectiformis, and abundance of Ostrea Weg- 

 D . ^- manniana and 0. incurva (often in bunches) in the 

 ^ "^ (^l ^ I lower part about 60 



About 105 



' The Zones of the Chalk in Hants, London, Dulau & Co., 1911. 



- Tlie Stratigraphy of the Chalk of Hants, London, Dulau & Co., 1912. 



