96 CARBONIFEROUS LAMELLIBRANCHIATA. 



The term Posidonia Gibsoni appears to be used in Lancashire for two very 

 different shells. Mr. Wild figured the specimen here shown by fig. 14, PI. VI, 

 under this term ; but in all collections specimens of P. minor, such as PI. VI, 

 fig. 15, also receive the same name. The latter is a much flatter and quadrate 

 shell. Mr. H. Bolton, who has carefully gone into this question, considers the 

 two forms to be perfectly distinct ; and though at first I was inclined to the view 

 that the latter represented P. isevis as it appeared when crushed and flattened in 

 the shales, and with much of the shell removed, and the periostracum only left, I 

 have come to the conclusion that he is correct, and I have therefore retained the 

 two species. 



Posidoniella Kikkmani, Brown, sp., 1841. Plate VI, figs. 16, 19 — 21. 



Catillus Kibkmant, Brown, 1841. Trans. Manchester G-eol. Soc., vol. i, p. 225, 



pi. vii, fig. 66. 



— costattjs, Brown, 1811. Ibid., p. 226, pi. vii, fig. 68. 



— obliquatt/s, Brown, 1841. Ibid., p. 226, pi. vii, fig. 69. 



— Kiekmani, Brown, 1849. Ulustr. Foss. Conch., p. 167, pi. lxvii, fig. 24. 



— costatus, Brown, 1849. Ibid., p. 167, pi. lxvii, fig. 25. 



— obliqtjatus, Brown, 1849. Ibid., p. 167, pi. lxvii, fig. 23. 



Specific Characters. — Shell small, moderately gibbose, produced downwards, 

 obliquely subquadrate. The anterior end is small, and produced at the anterior 

 superior angle into a small ear. The border is almost straight, and is directed 

 downwards and slightly backwards. The inferior border is very bluntly but 

 regularly curved, somewhat shorter than the superior. The posterior border is 

 straight above and rounded below. The hinge-line is straight in front, angulated 

 and depressed posteriorly, meeting the posterior border at an obtuse angle. The 

 umbones are swollen, small, not raised above the hinge-line, situated anteriorly, 

 but not terminal. There is often a bend in the posterior part of the hinge-line, 

 which gives an erroneous impression of a very short hinge-line, and consequently 

 subcentral umbones. The shell is evenly and obliquely swollen, the anterior 

 slope being the steeper ; above and behind, the valves are compressed into the 

 posterior angle. 



Interior. — Casts show that the anterior adductor scar is small, punctate, and 

 situated near the apex of the umbo ; the posterior small, and in the normal position 

 for the genus. The interior of the shell was smooth, but with concentric 

 undulations and obscure radiating strise over the posterior portion. 



Localities. — The Black Shales, below the Millstone-grit of Crimsworth Dean 

 and Horse-bridge Clough, Vale of Todmorden. 



