BIVALVIA. 31 



Localities. Barnack, Northamptonshire ; Ponton, Lincolnshire ; Culver Hill, on the 

 western side of Minchinhampton Common, in the Inferior Oohte. 



Pinna, Linn. 1758. 



Gen. Char. Shell longitudinal, wedge shaped, acute anteriorly, truncated and gaping 

 posteriorly; urabones straight, terminal and pointed; hinge lateral, linear, and without 

 teeth ; hgament marginal, linear, elongated, and partly internal ; muscular impressions 

 two, the anterior or byssal one minute, the posterior large. Substance of the shell thin, 

 structure fibro-lamellar, composed of two layers, of which the exterior one is fibrous, the 

 interior lamellar. 



Pinna ampla, Sow., sp. Tab. IV, fig. 14. 



Mytilus amplcs, 8ow. Min. Con., t. 7, 1812. 

 Pinna ampla, Goldfms. Pet., t. 129, f. 1. 



— — Deshayes. Lam. An. s. Vert., 2de ed., vii, p. 68, 1836. 



— — Bronn. Index Palseont., p. 977. 



Testa mytiliformi, mediocre gihhosd, costellatd ; costellis irregularihus, suhplanis, undii- 

 latis, plurimis dichotomis aut conjluentis et nodosis, striis transversis crehris et laminis 

 incrementi impressis .• umbonibus obtusis plerumque costellatis. 



Shell triangular, moderately gibbose, longitudinally costated ; costellse numerous, very 

 irregular, waved, for the most part bifurcated, confluent, knobbed and impressed with 

 transverse striae, which are very fine and closely arranged, and likewise by the laminae 

 of growth which are irregular. The hinge is straight, short, oblique, forming an angle of 

 45 degrees with the anterior border, the umbones are obtuse and usually costellated. 



The radiating little ribs are but slightly elevated; they are tolerably distinct upon the 

 anterior part of the shell, but posteriorly, where the folds of growth are larger, they become 

 very irregular, confluent, or vanish altogether, a change exactly similar to that exhibited in 

 the progress of growth of some recent Pinnas ; in fact, well-preserved specimens of Pinna 

 ampla are sometimes obtained, which are quite destitute of the longitudinal costeUae, and 

 retain only the folds of growth. The substance of the test is thicker than is observed in 

 recent shells of this genus. 



It occurs rarely in the shelly beds of the Great Oolite, but more frequently in the 

 Stonesfield slate of Gloucestershire. 



Localities. Minchinhampton Common, in Great Oolite; Sevenhampton Common, 

 in the Stonesfield slate ; Wiltshire, in the Cornbrash. 



In the Stonesfield slate of Stonesfield, Oxfordshire, and also in the Oolite of Mitford, 

 Somersetshire. 



