part 2] SYKIXGOTHYRIS AND SPIRIFEBI NA. 217 



Mutations of Spiriferina octoplicata. 



1. Spiriferina octoplicata, mut. D. (PL XIII, fig. 8.) 



This mutation is the typical Dibuuophi/llum-Zone form already 

 described, and represented by Sowerby's type-specimens. 



2. Spiriferina octoplicata, mut. 3, Vaughan. (PL XIII, fig. 9.) 



This is a small form possessing all the essential characters of the 

 later mutation, somewhat less highly developed. 



The area is lower, and the valves less convex than in the typical 

 form, than which, in consequence, the shell has a less inflated 

 appearance. It also differs from the later mutation, in that the 

 costse are less angular and are separated by shallower furrows, 

 while the concentric ornament is more regularly developed. 



Horizons and localities. — Middle Avonian, Horizons y to c, 

 in the South-Western Province. 



Remarks. — This mutation is the form defined by Vaughan 1 as 

 Sp. octoplicata, mut. ?. It differs from Sp. insculpta in the number 

 and size of its costse, which in the latter species are fewer in 

 number and larger in size. Sp. insculpta, moreover, occurs at a 

 higher horizon. 



It is readily distinguished from the Lower Avonian mutation of 

 Sp, perplicata which has five or six costse on each lateral slope. 



Sptriferina insculpta (Phillips). (Text-figs. 4<f& 5a, pp. 199, 

 211; PL XIII, fig. 11.) 



1836. Spirifera insculpta Phillips f27] p. 216 & pi. ix, figs. 2-3. 

 1844. Spirifera quinqae7oba M'Oo.v [20] p. 134 & pi. xxii, fig. 7. 



The original description is as follows : — 



• Spirifera insculpta. Bolland, Derbyshire. Cardinal area large ; the mesial 

 and two or three lateral folds very bold, acute, and strongly striated across.' 



Although the holotype of this species, which" is now in the 

 British Museum (Natural History) Geological Department, Gilbert- 

 son Collection, B 304, is fairly well depicted in Phillips's figures, 

 the name has been used in a loose way for small strongly-plicated 

 Spiriferoid shells from Devonian as well as from Carboniferous 

 rocks. Davidson's figures of the original specimen 2 are somewhat 

 misleading, and convey the impression that the shell is more 

 elongate than is actually the case. 



The holot} T pe is a large (for the species) mature specimen, and 

 is readily distinguished from adult specimens of Sp. octoplicata, 

 although the differences between young individuals of those two 

 species are less obvious. 



The essential characters of the species are as follows : — Shell 

 spiriferoid ; moderately-high curved area ; beaks small, but well 

 developed, and incurved in the pedicle- valve, prominent and in- 

 curved in the brachial valve. The cardinal extremities are rounded. 



1 Vaughan [35] p. 45. 



2 Davidson [4] pi. vii, figs. 52-55. 



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