100 CARBONIFEROUS LAMELLIBRANCHIATA. 



that Brown's shell was incomplete in front, and he dotted in an anterior lobe 

 which brings the specimen into his new genus. 



Avicula squanuda, Brown, probably belongs to the same species as A. tenua, 

 but whether or no they are examples of the species under discussion, there can be 

 little or no doubt that they belong to the same genus. The vertical distribution of 

 this species is of interest, as it occurs at different horizons in the Lower Coal- 

 measures, and also in the series of shales below the Millstone-grit of Lancashire 

 and North Staffordshire. 



The names Catillus rotundatus and G. tumidus occur on the original labels of 

 specimens in the Owens College Collection, from Todmorden. These do not occur 

 in any publication that I can discover, and in my opinion they are forms of the 

 species under description. 



Posidoniella variabilis {Brown, MS.), sp. nov, Hind. Plate VII, figs. 7 — 9. 



Mytilus variabilis, Brown, MS. Specimens in the Manchester Museum, Owens 



College. 



Specific Characters. — Shell equivalve, produced downwards, mytiliform, roughly 

 rhomboidal, very moderately convex in front and above. The anterior end is 

 obsolete, but the valve is bent on itself along the anterior border to form the 

 body-cavity. This border descends downwards and backwards, and is nearly 

 straight. In some specimens the bending of the valve is so acute that an anterior 

 surface, lanceolate in shape, is formed, and in others there is merely an expanded 

 border. The inferior border is obliquely rounded and compressed. The posterior 

 border is obliquely truncate, nearly parallel to the anterior ; and is straight above 

 and convex below. The hinge-line is straight, somewhat shorter than the 

 greatest transverse diameter of the shell, and makes an obtuse angle with the 

 posterior border. The umbones are small, gibbous, pointed forwards, and 

 terminal. Extending obliquely downwards and backwards from the umbones is a 

 swelling, which has a rapid rise on its anterior side, but posteriorly passes 

 gradually into the flattened and expanded posterior portion of the valve. 



Interior. — Though several specimens of casts are in the Owens College 

 Collection, no details of hinge- or muscle-scars are apparent. 



Exterior. — The surface is ornamented with fine, close, concentric lines of 

 growth, with here and there more prominent lines or obsolete sulcations. These 

 are all parallel to the borders, but are crowded in front. 



