248 R. M, Brydone — Chalk Zone of Holaster planus. 



with the typical T. lata-chalk of the Isle of Wight. The presence 

 of Bicavea rotula (paralleled inland by a specimen recorded at Arreton 

 by Howe himself 1 ) should alone be enough to deter anyone from 

 keeping this bed out of the zone of H. planus in order to attach it to 

 the zone of T. lata ; and you might search the typical T. /«^«-chalk 

 of the Isle of Wight or any other South English district I am 

 acquainted with for days without finding a single specimen of any 

 Polyzoon, while Polyzoa are of course thoroughly characteristic and 

 abundant features of the Senonian. The combined effect of these 

 points seems to make an overwhelming case for uniting the " Vectemis 

 bed" with the Senonian zone of HolasUr planus in preference to the 

 Turonian zone of Terebratulina lata. 



Once the Senonian character of the " Yectensis bed" has been 

 established the position of the "spurious Chalk Rock bed" becomes 

 an open question. The bed itself is so hard and the area of it exposed 

 in situ so small that palaeontology is not likely to give much help. 

 Its peculiar physical characters, striking as they are, cannot be relied 

 upon for identifying fallen blocks, as there is, at Culver Cliff at any 

 rate, a very similar bed in the zone of If. cor-testudinarium. It is to 

 be noted, however, that H planus is recorded from it, which is at any 

 rate some argument in the Isle of Wight for attaching it to the zone 

 of that fossil, and I have two specimens from it of the Polyzoon 

 Onychocella Lamarcki, which is no ordinary fossil of the T. lata-chalk.. 

 The lithological evidence is all in favour of the same course. There 

 is the widest difference between this bed and the typical T. lata- 

 chalk, while there is a considerable affinity between it and the 

 nodular and hard H planus-chalk above. I come, therefore, to 

 the conclusion that the " Vectensis bed" and the "spurious Chalk 

 Rock bed " should both be placed in the zone of Holaster planus, and 

 that the boundary between that zone and the zone of T. lata in the 

 Isle of Wight should be drawn at the violent change from a long 

 period of very uniform conditions of deposit which would seem to be 

 marked by the appearance of the " spurious Chalk Rock bed". 



It will be obvious that this conclusion cannot leave Dorset 

 unaffected. The Dorset coast sections embracing the base of the 

 zone of H. planus are, with the exception of the hopeless section in 

 Durdle Cove, singularly inconvenient for access and very limited 

 in area at the best. But it seems quite clear, from my own observa- 

 tions and those of Rowe, 2 that all along the Dorset coast a substantial 

 thickness of typical T. lata-chalk is followed by a sequence for all 

 practical purposes identical with that of the Isle of Wight; and the 

 objections to associating either the relatively very fossiliferous chalk 

 above the "spurious Chalk Rock bed" or the "spurious Chalk Rock 

 bed" itself with the alien chalk below are of the same character as 

 in the Isle of AVight, and, considering the nature of the exposures, 

 equally strong. 



The palaaontological and lithological evidence can obviously in the 

 Isle of Wight and appai-ently in Dorset be harmonized by taking the 

 presence of Holaster planus as the test of the beginning of the zone 



1 Op. cit., p. 263. 



2 Op. cit., pt. ii, 1901. 



