Vol. 67.] THE CARBONIFEROUS SUCCESSION IX COWER. 563 



(3) The slopes of the median fold pass gradually into the flanks, whereas, 

 in Sp. bisulcatus, the fold is sharply separated from the flanks by a 

 change in curvature and by two strongly marked furrows (to the 

 presence of which a the species owes its specific name). 



This species (or variant) is not uncommon in D.,_ ? near the 

 top of Oystermouth Quarry. Its small, convex, pinguis-lihe 

 appearance readily distinguishes it from the broad flattened Sp. bi- 

 sulcatus, var. oj/stennouthensis, which is the predominant form. 



Spirlfer wickexsis, sp. nov. (PI. XLI, fig. 7.) 



Compare the following forms figured in 'Davidson ' : — 

 'Sp. duplicieosta,' var., pi. iv, fig. 4 (from Park Hill). 

 ' Sp. grandicostatus,' pi. vii, figs. 8-11 (from Bolland). 

 Also compare : 



Sp. cameratus Morton, var. figured in Hall & Clarke, 'Palsont. New York/ 



vol. viii, pt. ii (1894) pi. xxxii, fig. 12, from ' Coal-Measures.' 2 

 See also Sp. striatas, var., in Q. J. G. S. vol. lxii (1906) p. 310 (from the 

 Curkeen Limestone). 



Description. — Cardinal angles often dissimilar, as in the 

 Bolland specimen cited above. 

 £ Median fold bounded by two deep furrows. 



i Ribs few, but tall, angular, and separated by deep furrows : the 

 striking and irregular manner in which they fork, so .that the 

 branches often diverge very widely, makes the species easy of 

 recognition. 



In so far as the type of ribbing is concerned, this is an old-age 

 character that can be arrived at by structural change within 

 distinct gentes. For example : — 



(1) The form in the Curkeen Limestone (Co. Dublin) is clearly a derivative 



from a striate stock, since it is connected with more normal members 

 of that stock by intermediate forms. 



(2) The American variant is derived from a stock having the ribs in 



fascicles, and has been evolved by the blending of each fascicle into a 

 single strong rib. 



(3) It is probable that in very many cases this type of ribbing has been 



arrived at from a variant of <%>. bisulcatus with coarse ribs, in which 

 the characteristic forking is already initiated near the margin. 



Hence this species is rather of the nature of a circulus than of a 

 true species ; it has, however, an important stratigraphical value, 

 since it is widely distributed and is as yet unknown below D 2 . 



In the Bristol area Sp. wiclcensis occurs abundantly in D 2 at 

 Wick (whence the specific name) and, somewhat rarely, at Wrington 

 and in the Avon section at the same level. 



In Gower it is a rare associate of the D 2 _ 3 fauna, and has been 

 found in Oystermouth Quarry. 



1 Davidson seems to have regarded the two grooves on the fold itself, to 

 which the fold owes its tripartite character, as the reason for its specific name. 

 Sowerby, however, states clearly that the character recorded in the name is the 

 separation of the fold from the flanks by two deep furrows. 



2 Hall & Clarke assign to the 'Coal-Measures' British specimens obtained 

 from Upper D (Lor instance, the types of the species : — Spirifer striatus, Beticu- 

 laria lineata, Martinia glabra, Prodv.ctus longispjinus). 



