SECTIONAL TRANSACTIONS.— C. 



355 



anomaly may, on the contrary, prove a valuable weapon in the study of the evolution 

 of the goniatites and in determining the conditions of deposition in the Upper 

 Carboniferous delta. The state of things is best exhibited in the following table 

 showing the south-westward variation in the two highest marine bands of the Mill- 

 stone Grit Series : — 



SW, 



Bolton & Horwioh 



Hasltnghkn 



Flag 

 Maiuni; Band 



HoLCOMBK 



BnooK 

 Marine Band 



KOSSI'.NDALF. N. 

 G. listeri 



G. listeri 



G. cumbriense 



NE. 



TODMORDEN ct BlaCKBUUN 



I G. cumbriense | 



G. cronulatum 



G, cumbriense 



G. crenulatum 



G. crenulatum 



G. cumbriense (small, 

 rare) 



G. crenulatum 



G. of. crenulatum (rare 

 G. cancellatum 



Same, 



BoxroMLEAF. 



G. cancellatum 

 G. cancellatum 



G. cancellatum 

 G. cancellatum 



R. ret"" mut. y 



R. ret" mut. y 



Befl failing North 



Now it is highly improbable that the successive members of a faunal sequence 

 can, under uniform conditions, occupy portions of the same sea not more than twenty 

 miles apart at one and the same time. One must rather, in view of the species always 

 maintaining the same order of development, regard them as absolute indices of the 

 passage of time, and if so there is no escape from the conclusion that the Haslingden 

 Flag Slarine Band was not contemporaneous in the north-east and south-west. The 

 idea is not a new one and analogous phenomena are demonstrable in almost every 

 highly fossiUferons formation. It is now proposed to introduce the term diachronous 

 to describe a bed having such relations to the zonal succession. The word is self- 

 explanatory and avoids a cumbersome circumlocution. 



It will be noticed that the essential peculiarity of the Haslingden Flag Marine 

 Band is that it drops zonal forms at the base as it takes others on at the top. The 

 relations of the Holcombe Brook Marine Band are somewhat different. When traced 

 •outhand west it takes on zonal forms both at top and bottom. It is diachronous as 

 regards its upper and lower limits, but synchronous as regards its centre. The 

 y-bed below behaves similarly and actually dies out to the north-east. The sea in 

 which the two last-mentioned marine bands were laid down advanced from the 

 south and west and withdrew towards the same points of the compass ; that Ln which 

 the Haslingden Flag Marine Band was deposited advanced from the north-east and 

 withdrew to the south-west. 



16. Miss Sibyl M. Hampton and Prof. H. L. Hawkins. — A Revision of 

 the Ecliinocystoida. 



Newly acquired material of the problematical genera Echinocystites and Palceodiscus 

 has made possible some additions to our knowledge of the structure and affinities of 

 these Silurian Echinoidea. It appears that Echinocystites is a thoroughly established 

 and specialised Echinoid, and that in most respects it seems akin to the Lepidocentridse. 

 The new evidence points to its endocyclic character ; while the great development 

 of its madreporite may be correlated with the complexity of its ambulacra as indicating 

 Tigorous use of the podia. 



In the case of Paloeodiscus it has been possible only to add a few details, but the 

 discovery of several specimens which show the inner and outer aspect of the adoral 

 surface helps in the interpretation of older material. The existence of perfectly 

 typical Echinoid pore-pairs in the ambulacral plates (both coronal and peristomial) 

 i.? confirmed ; and indications of structure can be detected in the so-called ' anal 

 area.' Practically every detail of the lantern can be studied in an undistorted state. 



In neither genus does the new material afford the slightest suggestion that there 

 can be two series of ambulacral plates ; nor has re-examination of most of the 

 material in older collections lent any support to that hypothesis. 



A a2 



