220 DE. F. J. NOETH OS [vol. lxxvi,. 



Mutations o£ Spiriferina perplicata. 



1. Spieifeeina peeplicata, mut. T>. 



This is the form represented by the holotjpe of the species 

 (PL XIII, figs, letr-lc). 



2. Spieifeeina peeplicata, mut. K. (PI. XIII, fig. 10.) 



1905. Spiriferina cf. octoplicata Vauglian, ' Bristol Paper' [33] pi. xxvi, fig. 6 ; 



and ' Burringtoi) Paper ' [28] p. 384. 

 1915. Spiriferina octoplicata mut. K, Vauglian, 'Belgian Paper' [35] p. 44 



& pi. vii, fig. 1. 



This mutation has all the essential characters of the species, as 

 already enumerated, but it differs from the tj^pical (D) form in the 

 following respects : — The cardinal area is lower and the brachial 

 valve less convex • the shell is, in consequence, less gibbose. 



Pedicle- valve. — The beak is more sharply incurved ; the cost* 

 on either side of the sinus are rather more conspicuous than the 

 lateral costai, so that the sinus is sharply defined ; this character is 

 still noticeable, although less obvious in the later mutation. 



Brachial valve. — The fold is slightly flattened anteriorly, and 

 the costse are separated by somewhat shallower furrows. 



Horizons and localities. — In the South- Western Province 

 this mutation occurs in the Cleistopora Zone and at Horizon fi. 

 In the Avon Section it reaches its acme at the top of K 3 , but in 

 the Burrington Section it is rare at that horizon ; at Miteheldean 

 it occurs in K. 



Remarks. — This mutation is the form cited by Yaughan as 

 Spiriferina cf . octoplicata in his earlier (that is, ' Bristol ' and 

 'Burrington') papers, and as Sp. octoplicata mut. K in his 'Belgian 

 Paper.' In the 'Burrington Paper' (pp. 384-85), he also compares 

 it to Sp.peracuta Be Koninck, which is abundant in Z, of Belgium. 

 In the absence of an examination of the type-specimen of 

 Sp. peracuta, a comparison between that species and the form under 

 discussion is not possible ; but, judging from De Koninck's 1 and 

 Vaughan's 2 figures of typical specimens, the two are distinct, 

 Sp. perplicata having more costse and a relatively-larger median 

 fold than the other form. In any case, this does not affect the 

 conclusion arrived at concerning the relationship of Sp. perplicata 

 mut. K, because, even if it should eventually prove to be identical 

 with De Koninck's species, it would still be the Lower Avonian 

 representative of Sp. perplicata . 



The two following forms figured by Portlock probably belong 

 to this species : — 



(1) The obscure specimen described as Spirifera partita,'' and now in the 

 Museum of Practical Geology (Reg. No. 28427). Owing to its 



1 Ann. Mus. Roy. Hist. Nat. Belg. vol. xiv, pt. 6 (1887) pi. xxii, figs. 56-61. 



2 Vaughan [35] pi. vii, fig. 2. 



3 J. Portlock, 'Geol. Rep. Londonderry, &c.' 1843, p. 567 & pi. xxxviii, 

 fig. 3 ; see also Davidson [4] pi. vii. figs. 60-61. 



