310 DR. A. VAUGHAIS" ON THE EATTXAL SUCCESSION [May 1906, 



transverse Derby a occurs somewhat abundantly ; it has strong con- 

 centric ruts, but never exhibits .the remarkable contortion of Derby a 

 anomala. 



LEPTiENA Cf. DISTOETA, J. SoW. 



The dependent margin of the pedicle-valve is practically smooth, 

 narrow, and deeply sinuated. The double- valved shell is biconvex, 

 since the margin scarcely projects below the brachial valve. 



This form occurs rarely in D 2 of the South- Western Province. 



Spirifers. 



Gens of Spirieee bistjlcatus, Sow. 



Pigs. 13 & 14 in pi. vi, Davidson, 1 represent very closely the com- 

 monest type found in the Cyathaxonia-Beds of the Rush sequence. 

 The flank-ribs, however, increase very markedly in strength as they 

 approach the fold or sinus, after the pattern of Sp. grandicostatus, 

 M'Coy. 



The same form occurs abundantly in D of Gower (South- 

 Western Province). 



Gens of Spieifee steiatus (Martin). 



"Without undertaking the much-needed revision of the striate 

 Spirifers, it is necessary to enumerate the most obvious of the 

 characters exhibited by Martin's type-figure. 



Broad, rectangular area, forming the widest part of the shell. 

 Small, sharp, bicrested beak, recurved like a hook over the top 

 line of the area. Both valves completely and sharply ribbed ; the ribs 

 are very numerous, and arranged in pairs or groups, radiating from 

 the umbo of each valve ; the ribbing on fold and sinus is on the 

 same pattern as that of the flanks ; the margin of the fold is non- 

 truncate. 



The Spirifers figured in Davidson, op. cit. pi. ii, and pi. iii, figs. 2, 

 4, 5, are probably all members of this gens ; I should also include 

 here such forms as pi. iv, fig. 4, and pi. vii, figs. 8 & 9. 2 Spirifer 

 planicosta, M'Coy, should, I think, be excluded from this gens, and 

 yet not included in the gens of Sp. trigonalis. 



The predominant form in the Curkeen Limestone is very similar 

 to the forms illustrated in Davidson, op. cit. pi. ii, fig. 17, and in 

 pi. iii, fig. 5. 



A well-marked variant, which resembles Davidson, op. cit. pi. vii, 

 fig. 8, and pi. iv, fig. 4, is remarkable for its few very coarse and 

 angular ribs, which split into groups of shorter ones as they traverse 

 the valves. The same variant occurs in D 2 of the South- Western 

 Province, and is there also associated with a normal member of the 

 gens. 



1 ' Monogr. Brit. Foss. Brachiop.' (Palasont. Soc.) pt. v, 1858-63. 



2 The inclusion of a particular form in a certain gens must be decided by 

 the constancy of its association (or time-sequence) with a recognized member 

 of the gens, and not by the possession of one or more striking characters which 

 may have been artificially set out as an exclusive test of membership. 



