Yol. 6$.~] BASEMENT-BEDS OF THE BRISTOL COALFIELD. 461 



along the ventral border, and by radiating ribs passing from the 

 posterior side of the umbo to the hinder margin. In this region 

 the ribs are well marked. Traces of the distal portions of feeble 

 ribs are continued down on to the hinder ventral border. 



Posidoniella l^ivis, Brown. 



This species occurs in association with small gasteropods, and is 

 by no means rare. The shell, where present, shows coarse con- 

 centric corrugations, while internal casts show an ill-defined 

 longitudinal ribbing. The specimens exhibit the most typical form 

 of the species. 



Posidoniella Kirkmani, Brown. 



This species is represented by dwarfed individuals with a much- 

 wrinkled periostracum. They are considerably rarer than P. Icevis, 

 and seldom exceed 4 millimetres in length. 



Posidoniella minor, Brown. 



This species is much rarer than either P. Icevis or P. Kirkmani. 



Schizodus antiqjjtjs, Hind. (PL XXX, fig. 9.) 



Fragments of 1 this species are fairly numerous, but nearly always 

 in the condition of internal casts. The shell, where present, is of 

 a strong brown colour, very thin and friable, marked by concentric 

 furrows, which become stronger as the margin of the shell is 

 approached. 



Aviculopecten gentilis, Sowerby. 



A fragment of a left valve and a cast of the right valve were 

 found of this species, the latter belonging to a much smaller shell 

 than the former. 



Pterinopecten carbonaritjs, Hind. 



A single right valve, somewhat incomplete. The ribs appear 

 slightly monilated towards the inferior margin of the shell, and 

 there is an absence of the smoothness in the region of the posterior 

 ear which is usually perceptible. This specimen was submitted 

 to Dr. Wheelton Hind, who determined the species. It has since 

 broken up into fragments during the process of drying. 



Pterinopecten paptracetjs (Sowerby). 



The occurrence of this species is of considerable interest, as its 

 range is so thoroughly well known. Dr. Hind has drawn atten- 

 tion 1 to its range from the top of the massif of the Carboniferous 

 Limestone to high up in the Coal-Measures. The latter limit is 

 somewhat indefinite ; but there is no doubt that the species is of 

 rare occurrence above the Lower Coal-Measures, the chief of the few 

 levels in the Middle Measures in which it occurs being the marine 



1 ' Monogr. Carbonif. Lamellibr.' vol. ii, Palseont. Soc. (1903) p. 53. 



