562 MK. E. E. L. DIXON AND DR. A. VAUGHAN ON [Nov. I9II. 



SPIRIFERIDS. 



Spirifer. 



Spirieer bisulcatus Sow., var. oystermouthensis, no v. (PI. XLI, 

 figs. 5 a & 5 6.) 



This variant differs from Davidson's figure 1 of the type of the 

 species (' Davidson,' pi. vi, figs. 6-9) in having : — 



(1) Broad scarped ribs and a gently convex mesial fold, but little differ- 



entiated from the flanks ; 



(2) A flatter and more transverse form ; and 



(3) The cardinal region almost smooth. 



It is extremely abundant in D 2 _ 3 of Oystermouth, but rare in 

 D a _ 3 of Bishopston. 



Comparisons. — Spirifer calcaratus M'Coy 2 (PI. XLI, fig. 5 c) 

 differs in its spine-like wings, unequally developed on the two sides. 

 This species is common in l) 2 _ 3 of Pateley Bridge (Yorkshire). 



Spirifer bisulcatus Sow. var., Q. J. G. S. vol. lxiv (1908) pi. 1, 

 figs. 3 a & 3 b, differs in its small size and want of bilateral 

 symmetry ; it is common in Lower P of Loughshinny (Co. Dublin). 



Ornament. — The intersection of radial and concentric orna- 

 ment produces the characteristic ; twilling ' shown in fig. 5 b. This 

 type of ornament is common to all the forms mentioned above. 



Spirifer near increbescens Hall. (PI. XLI, fig. 6.) 



Compare Sp. increbescens in Hall & Clarke, ' Palaeont. New York,' vol. viii, 

 pt. ii (1894) pi. xxx, figs. 27-30 (specimen from Chester Limestone). 



Compare also the type of Sp. bisulcatus Sow., as refigured bv Davidson (pL vi, 

 figs. 6-9). 



Our species only differs from Sp>. increbescens in its smaller size 

 (27 mm. wide as against 38 mm.), and probably also in the smaller 

 degree of differentiation of the median fold from the flanks. 



The shagreen-like ornament is also not so conspicuous as in 

 fig. 30 of the American species. 



The differences from the type of Spirifer bisulcatus Sew. are 

 more numerous, namely : — 



(1) The smaller size. 



(2) The tall, angular median fold is composed of a narrow, central, grooved 



rib and two flat broad ribs, which form the steep slopes of the fold on 

 either side ; the valve-intersection is strongly deflected into a tall 

 domed arch. In Sp. bisulcatus the median fold is gently and con- 

 tinuously convex from flank to flank, and the deflection is semicircular. 



1 There is considerable doubt whether the imperfect specimen in the 

 Sowerby Collection at the British Museum (Natural History) is actually the 

 holotype of Spirifer bisulcatus Sow. ; if so, both Sowerby and Davidson have 

 erroneously restored the mesial fold. 



2 ' Synopsis Carb. Foss. Ireland ' 1844, pi. xxi, fig. 3 ; refigured in 'David- 

 son ' pi. vii, fig. 4. 



