CARBONICOLA SIMILIS. 77 



borders. This compression in many cases results in a sloping smooth surface, 

 slightly concave from side to side. 



The lunule is elongated and narrow. External ligament short and erect. 



Valves evenly convex ; in some cases flattened, in others more cylindrical. 

 Shell thin. Lines of growth very fine, arranged concentrically. Periostracum 

 thin ; much wrinkled and folded towards the circumference of the shell. 



Interior. — The posterior adductor-muscle scar is situated along and just above 

 the ridge which passes upwards into the umbo. Hinge-plate as in G. robusta, 

 PI. II, fig. 1 a. 



Dimensions : 











PL XI, fig. 6. 



fig. 8. 



fig. 9. 



Antero-posteriorly 



. 30 mm. 



32 mm. 



27 mm 



Dorso-ventrally 



16 mm. 



20 mm. 



14 mm 



Laterally 



6 mm. 



13 mm. 



7 mm 



Localities. — A thin band of ironstone above the Cockshead Rock. Hulme 

 Colliery, Longton. Horizon of Banbury Coal, Bucknall. South-west of Mow Cop 

 on the same horizon. 7-foot — Banbury? (Talk-o'-th'-Hill). All in the lower 

 measures of the North Staffordshire Coal-field. South Staffordshire, above the 

 Brooch Coal. Scotland — Lurgar. Springshill Colliery, Ayrshire. 



Remarks. — From the figures and description I have no doubt that the species 

 under notice is that figured by Captain Brown under the names of Pachyodon 

 and TJnio similis, the figures representing an adult form ; while in his figures of 

 Pachyodon and TJnio nanus a young form is depicted. Prom the series of 

 specimens which I figure in PL XI, figs. 6 and 7, 9 — 13, and 15, from the horizon 

 of the Brooch Coal, Dudley, it appears that the characteristic shape of the adult 

 shell is not possessed by the young, although the oblique line passing from the 

 umbones backwards to the lower angle is always present, together with the 

 characteristic compression of the shell above this line. In young shells, however, 

 the posterior end is truncate and the inferior extremity acute ; but in process of 

 growth the shell expands upwards and backwards at the posterior superior angle 

 more rapidly than it does below, so that the posterior end becomes expanded in 

 a dorso-ventral direction, and the end of the shell itself becomes bluntly rounded. 



There is also at the same time a greater expansion circumferentially, so that 

 the adult shells are much less convex comparatively than the young. These 

 changes by growth account for the fact that Captain Brown made two species of 

 what I consider I have shown to be only one ; and, indeed, if it were not that I 

 fortunately was able to study a series of shells in all stages of growth, I have no 

 doubt that I should have separated the adult and young forms into two different 

 groups. 



From my observations in the North-Staffordshire Coal-field I consider that 



