POSIDONIELLA LiEVIS. 95 



Exterior. — The surface is covered with concentric folds and lines of growth. 

 Periostracum wrinkled. 



Dimensions. — Fig. 12, PI. VI, a right valve, partly decorticated, measures — 

 Antero-posteriorly . . . .18 mm. 



Dorso-ventrally . . . .18 mm. 



Elevation of valve . . . .5 mm. 



Localities. — In the roof of the Bullion-coal Foot-mine ; Colne and Sholver, 

 near Oldham, Starring, Dearnly. Over the Gannister coal, Pimbo, near Wigan. 

 In a band with Avicidopecten papyraceus, Lower Coal-measures, Froghall, North 

 Staffordshire. In the shales below the Millstone-grit ; of the Coombe Valley, near 

 Leek, Crimsworth Dean, Hebden Bridge, and High Green Wood, Todmorden. 



Observations. — A number of specimens of this species, in different stages of 

 growth and states of preservation, have been described and figured by Captain 

 Brown (op. cit.) under several different names. Many of them were obtained from 

 High Green Wood, Todmorden ; and, strange to say, these names have escaped the 

 notice of all cataloguers and palaeontologists. The originals of these specimens 

 are nearly all fortunately preserved in the Manchester Museum, Owens College, 

 and I am permitted to re-figure them. 



Mr. Salter, when working up the palaeontology of the Lancashire coal-fields for 

 the ' Memoirs of the Geological Survey,' uses the names Posidonia Gibsoni, 1 Brown, 

 and Posidonia laevigata, 2 Brown, but gives no clue as to their origin. Brown in 

 neither of his papers quoted above mentions these names, and the use of them 

 will consequently have to be discontinued. 



The original of Catillus Isevis is evidently a specimen which has lost its 

 periostracum and outer layer, and is therefore decorticated. This accounts for 

 the different appearance of this specimen from others in a more perfect state, and 

 doubtless gave origin to the specific name. When in a better state of preservation, 

 as in PI. VI, fig. 13, the shell is seen to possess well-marked concentric undulations. 



The animal of Posidoniella laevis was gregarious, and many specimens are 

 found together on slabs showing the shell in all stages of adolescence. This fact 

 has very largely weighed with me, together with the disappearance of the original 

 examples, in considering these forms as all belonging to one species. Apparently, 

 too, all Captain Brown's forms occurred together. 



Fig. 24, PI. VI, shows a large number of examples, in all stages of growth, 

 attached to a piece of fossil wood. It would therefore appear that this species 

 was byssiferous. This block, obtained by Mr. G. Wild, of Ashton-under-Lyme 

 from the Bullion Coal in the neighbourhood of Colne, was in his cabinet, and 

 by his kind permission I was able to figure the specimen. 3 



1 ' Geology of the Country round Wigan,' p. 35. 



2 ' Geology of the Country round Oldham,' p. 62. 



3 It is now in the Manchester Museum, Owens College. 



