Vol. 63.] BASEMENT-BEDS OF THE BRISTOL COALFIELD. 457 



four horizons, in great quantity and of all sizes, from very small fry 

 to forms 9 millimetres long. 



Most have a high, polished, and prominent umbo, which is 

 frequently notched or fractured along the medial line. In many 

 the periostracum is of a rich dark-brown colour, and readily 

 detachable as a thin film. 



Certain bands in the shale contain nothing else but Lingulce, 

 which are dotted over every bedding-plane in great profusion. 



OftBicuxoiDEA nitida, Phillips. 



This species is represented by numerous specimens, which are 

 usually very small. 



Ventral valves fairly numerous, circular, flat, and marked with 

 fine concentric lines or ridges, easily distinguishable in the smallest 

 individual. The foramen in one case is markedly ovate, and 

 surrounded by a raised margin, which is most strongly developed 

 near the centre of the valve. 



The species occurs usually in association with Lingula mytiloides 

 and a few unknown ostracods. 



Orthothetes cf. crenistria, Phillips. 



Pour shell-fragments were found, all of which are apparently 

 referable to this species. 1 



' Closely-similar forms occur in the Uppermost Avonian of the South-Western 

 Province (Gower), and are associated with Cyathaxonia in the Midland 

 Province at Congleton Edge.' [A. V.~\ 



Derbya senilis, Phillips. 



This determination is based upon the type of ribbing and broad 

 irregular wrinkling of the shell, on two very poor fragments. The 

 species is mainly characteristic of the Upper Dibunojphyllum-Zone 

 of the Avonian. 



Chonetes cf. hardrensis, Phillips. (PI. XXX, fig. 4.) 



These specimens are difficult to determine, owing to the way in 

 which the matrix clings closely to the shell, and fills up more or 

 less completely the interstices between the ribs. The biggest 

 specimen has a diameter of 12 millimetres along the hinge-line. 

 The general characters agree closely enough with those of Chonetes 

 hardrensis, and less so with those of Ch. Buchiana, which has 

 coarser and fewer ribs. The biggest specimen shows the basal 

 portions of ten hinge-spines. The ribs are small, bifurcate high 

 up above the middle of the valve, and are somewhat angular and 

 very numerous. The pedicle-valve is evenly convex, and shows no 

 trace of median sulcus. 



1 Quotations are from notes supplied by Dr. A. Vaughan. 



