Dr. J. W. Gregory — A new Liassic Echinoid. 317 



having been explained as due simply to the dynamic effects which 

 have accompanied igneous intrusions. This may perhaps be 

 regarded as the high-water mark of enthusiasm in this direction, 

 and would find but few followers now, extreme views as to dynamic 

 metamorphism being apparently much less in favour than they were 

 a short time since, and many of us who were to a considerable 

 extent believers having become rather more sceptical. 



But dogmatic opinion in an opposite direction is quite as much to 

 be deprecated. It is always desirable to bear in mind that though we 

 may have to consider any number of cases of highly-developed 

 crystalline slates, phyllites, and schists, demonstrably derived from 

 sedimentary deposits, and equally demonstrably affected in a high 

 degree by dynamic action, we are not yet, therefore, in a position to 

 assert that the crystalline and mineralogical development of those 

 rocks is caused by this dynamic action. It may conceivably be due 

 to the contact-action of concealed igneous masses, or it may be due to 

 the other agency which we are considering. The dynamic action 

 may have accompanied either of these, and possibly assisted them, 

 but may have had really very little, if any, share in the causation 

 of the effects with which we are now concerned. And on the other 

 hand, it is perhaps still more important to give due weight to the 

 evidence of those cases in which we have dynamic action of the 

 most intense description, and yet find that the rocks so affected, 

 while chemically and mineralogically closely resembling those just 

 considered, are very much less developed, and have, indeed, in this 

 special direction very little to show for the work done on them. 

 [To be continued in our next Number.) 



VI. — Arch^odiadema, a New Genus of Liassic Echinoidea. 

 By J. W. Gregory, D.Sc, F.G.S. 



OWING to the absence from England of any marine fossils of 

 Triassic age — except the few obscure shells described by Mr. 

 E. B. Newton 1 — the Lias yields the first English representatives of 

 the Neozoic Echinoidea. The fauna is not rich in species, and as 

 a rule the specimens are small ; but it is of interest, as its members 

 are primitive in character, and as they foreshadow many of the main 

 lines of evolution followed by the rich Echinoid faunas of the Oolites. 

 Some months ago Mr. Beeby Thompson showed me some specimens 

 from the Upper Lias of Northamptonshire, which form an interesting 

 addition to the primitive types of Liassic Echinoidea ; they belong 

 to a new genus, which is the simplest known form of the Diadema- 

 tinae, and occupies the same relation to the remaining members of 

 that subfamily that Eodiadema occupies to the Orthopsinae. 

 Arch^odiadkma, nov. gen. 

 Diagnosis. — Liadematidae DiadematinaB 2 with the following 

 characters : — 



1 R. B. Newton, " Note on some Molluscan Remains lately discovered in the 

 English Keuper " : Journ. Conch., vol. vii, Nos. 11 and 12, 1894-5, pp. 408-13. 



2 P. M. Duncan, " Revision of the Genera and Great Groups of the Echinoidea" : 

 Journ. Linn. Soc, vol. xxiii, 1890, pp. 5S-9. 



