Vol. 6$.'] BASEMENT-BEDS OF THE BRISTOL COALFIELD. 459 



Chonetes Laguessiana, de Kon. (' Monographic du Genre Chonetes ' 

 pi. xx, fig. 6), differs entirely from our shell, in that the curvature 

 is uniformly convex, and the ribhing so fine that (even when 

 magnified two diameters) it is scarcely visible. It is unfortunate 

 that Davidson adopted this extreme species as the general index 

 of the complex which includes the greater number of the small 

 Carboniferous Chonetes. I have followed M'Coy in regarding Ch. 

 hardrensis, Phill. {pars) as a more satisfactory index for the 

 distinctly-ribbed small Chonetes. 



Closely-allied forms are common in the Cyathaxonia-Zone of all 

 British localities ; for example, Dr. Wheelton Hind figures a nearly- 

 related shell in Proc. Eoy. Irish Acad. vol. xxv (1905) p. 106 & 

 pi. v, fig. 2. 



Productus cf. antiquatus, Sowerby. 



One fragment only was found of a large wrinkled semireticulate 

 Productus, of which there is no record above the Cyatlxaxonia- 

 Zone. 



Productus concinnus, Sowerby, mut. (PI. XXX, figs. 5 a & 5 b.) 



Mutation towards Productus muricatus, Phillips. 



A short, coarsely-ribbed, spinose mutation, which is convergent 

 on Productus longispinus. 



Very similar forms are common in the Cyatha.vonia-'Beds, and a 

 nearly-related, but more elongate form abounds in the shales of the 

 Millstone Grit of Pateley Bridge ; Productus muricatus is quoted by 

 Davidson from as high as the Millstone Grit of Scotland. 



Ambockelia aff. Urii, Fleming. (PI. XXX, figs. 6 a & 6b.) 



The specimens show, somewhat imperfectly, the short spines 

 which roughen the surface of the valves. The pedicle-valve is 

 gently convex, wider than long, and with a shallow median sulcus 

 narrowing towards the beak. The surface is marked by faint 

 radial ribs, and fine concentric lines of growth in close order. The 

 greatest width is about 10 millimetres, the length 6 mm. The 

 brachial valve is slightly concave, smooth anteriorly, and shows 

 faint lines of growth along the margin. 



The specimens differ from Ambocoelia carbonaria, Hind, 1 in their 

 less tumid character and the presence of faint radial ribs. A. Urii, 

 with allied forms, is known to range from the Uppermost Avonian 

 to the Permian. 



Lamellibranchiata. 

 Xucula ^qualis, Sowerby. (PI. XXX, figs. 7 a-7 e.) 



This species is extremely abundant, and invariably well preserved. 

 The interior of a left valve shows the closely- set teeth most 

 numerous posterior to the umbo. 



1 'On the Palaeontology of the Marine Bands in the Coal-Measures' Quart. 

 Journ. Geol. Soc. vol. lxi (1905) p. 531 & pi. xxxv, figs. 7-7 a. 



