Transactions of the Society. 



III. Mode of Occurrence and Zonal Distribution. 



In this country Porosphmra is met with most commonly in 

 the Chalk cliffs of the coasts of Yorkshire, Kent, Sussex, Isle of 

 Wight, Dorset, and South Devon. It is by no means uniformly 

 distributed in the Chalk, for whilst in some beds and horizons 

 specimens are very plentiful, in others they are rare or apparently 

 absent altogether. They are usually seen slightly projecting from 

 the weathered face of the cliff, from which they can readily be 

 picked out with a blunt knife or chisel. When naturally weathered 

 out of the cliff and fallen to the beach, they are soon worn smooth 

 and their surface characters are partly obliterated. As a rule, 

 they are seldom found in inland areas, either in Chalk pits or on 

 downs and other places where the Chalk outcrops at or close to 

 the surface ; but their apparent scarcity in these positions may be 

 due to their generally small size and to their close resemblance 

 to rolled fragments of Chalk. As an instance of their occurrence 

 in such places when carefully searched for, I may mention that 

 within the limits of a moderately-sized garden situated on the 

 slope of a Chalk clown at Croydon, Surrey, I have during the last 

 sixteen years picked from the surface soil 632 specimens of dif- 

 ferent forms of Porosphwra which have all been derived by slow 

 weathering from the underlying Chalk. 



With the object of tracing the development of Porospliaira at 

 various horizons in the Chalk, the collections made by Dr. Eowe 

 and by myself from each zone and locality have carefully been kept 

 separate, and in the list below details are given of the number and 

 range of size of each species from the respective zones, which will 

 show approximately the relative proportions of the different forms 

 which occur together. About two-thirds of the total number of 

 specimens in this list have been obtained by Dr. Eowe. 



It will be seen from this list that the earliest appearance of 

 Porosphccra in the Chalk of this country is in the zone of Holastcr 

 sulglobosus at Dover and at Durdle Cove, Dorset. There are only 

 very few specimens present on this horizon, and they all belong to 

 one species, P. Woodwardi, Carter, which, so far, has not been found 

 in any higher zone. 



In the next higher zone of PJiynchonclla Cuvieri, the pre- 

 dominant species, P. globularis, Phillips, occurs for the first time — 

 it is represented by only a few small specimens ; with it are some 

 diminutive forms of P. arrecta. P. globularis is more abundant in 

 the succeeding zone of Tcrcbratulina gracilis, but the specimens 

 are still small generally ; in this zone the limpet-shaped P. patclli- 

 formis is first met with. 



At the base of the Upper Chalk, in the zone of Holastcr planus, 

 the thimble or inverted cup-shaped P. pilcolus makes its first 

 appearance. 



