J. A. Thomson — Recent and Tertiary Rhynchonellids. 387 



greater Cardiff-Cowbridge anticline. The occurrence of Silurian 

 rocks beneath the surface at the localities already cited does not 

 indicate one continuous Silurian mass, immediately beneath the 

 Trias, extending from Pen-y-lan on the north to the Bristol Channel 

 on the south, and from Rumney on the east at least as far as Ely on 

 the west. There are probably several small oval areas which owe 

 their existence to pre-Triassic folding and denudation. In conclusion, 

 the writer would express his thanks to Professor T. Franklin Sibly 

 for his advice while the work was in progress. 



II. — The Genera oe Recent and Tertiary Rhynchonellids. 

 By J. Allan Thomson, M.A., D.Sc, F.G.S. 



OWING to the marked persistence of general external form and 

 internal characters in the Rhynchonellidse, it is necessary in this 

 family to seize on slighter differences for the foundation of genera 

 than is elsewhere necessary or advisable. 1 The object of this paper is 

 to point out the characters that have been used and others which may 

 advantageously be introduced in the discrimination of the genera 

 of Recent and Tertiary representatives of the group. These are not 

 at present distributed by authors amongst the genera Rhynchonella, 

 Fischer; Acanthothyris, d'Orbigny ; Hemithyris, d'Orbigny ; Crypto- 

 pora, Jeffreys (syn. Atretia, Neatretia) ; Frieleia, Dall ; and Basiola, 

 Dall. 



Surface Ornament. 



Buckman in a recent paper 2 divides Jurassic Rhynchonellids into 

 three main series — Zaves, Capillatce, Ornata. " Laves are smooth 

 and develop ribs directly on a smooth stage ; Capillatce have hair-like 

 lines (strice) and then may develop ribs ; and Ornatce have additional 

 ornament like imbrication, or spines." 3 In making this statement 

 Buckman cannot be taken to mean that the three series obtained 

 by a consideration of surface ornament are natural groups, equivalent 

 for instance to sub-families, since he has already recognized smooth, 

 ribbed, imbricate, and spinous species in a single genus, Hemithyris. 

 Schuchert 4 and others have placed the Recent Japanese spinous species, 

 Rhynchonella doderleini, Davidson, under Acanthothyris solely on account 

 of the possession of spines, and Buckman has strongly criticized this 

 course. " Spinosity is in itself not a generic character, it is only 

 a stage of development to which various stocks attain. R. doderleini 

 can have no connexion with the Jurassic species of Acanthothyris ; and 



1 Cf. J. Hall and J. M. Clarke, " An Introduction to the Study of the 

 Brachiopoda, etc." : 47th Ann. Bep. New York State Museum, 1894, 

 pp^ 1016-17. 



2 S. S. Buckman, "The Brachiopoda of the Namyau Beds of Burmah : 

 Preliminary Notice" : Bee. Geol. Surv. India, vol. xlv, pt. i, pp. 75-81, 1915. 



3 " The type of Rhynclionella, R. loxia, Fischer, is one of the Capillata. 

 The acuta group, which so resembles it, belongs to the Lceves, and so must be 

 removed. The result is that Rhynchonella, which once covered hundreds of 

 species from Ordovician to Becent, must now be confined, so far as present 

 knowledge goes, to one species, R. loxia " (Buckman, loc. cit.). 



* C. Schuchert, Brachiopoda in Zittel, Textbook of Palaeontology, translated 

 by C. R. Eastman, 2nd ed., 1913, p. 400. 



