26 MOLLUSCA FROM THE GREAT OOLITE. 



It may be conveniently divided into the following groups : 



a. Species with the umbones divergent, having between them a triangular area, borders 



of the valves rounded, lunule distinct and gaping. 



b. Umbones approximate, borders of the valves rounded, lunule small and closed. 



c. Species more flattened and elongated, or chisel shaped, the borders of the valves 



truncated, lunule gaping, its borders folded backwards. 



d. Umbones approximate, borders of the valves truncated anteriorly, lunule closed. 

 Our Great Oolite species will be found to contain examples of each of the foregoing 



groups. 



LIMA DUPLICATA, Sow., sp. Tab. Ill, fig. 6, 6 a. 



PLAGIOSTOMA DUPLICATA, Sow. Min. Con., t. 559, f. 3, 1827. 



? PECTENOIDES, Zieten. Wurtt., p. 92, t. 69, f. 2. 



LIMA ALTERNICOSTA, Buviffnier. Geol. de la Meuse, p. 22, 1. 18, f. 11 13. 



Testa convexd oblique-ovatd, antice abrupti truncatd ad cardinem angustatd, postici 

 rotundatd, auriculis parvis subaqualibus ; costis 25 28 angulatis, carinatis, sulcis confor- 

 mibm in imo sulcorum costd minima ornatis, costis tenuissime transverse striatis. 



Shell convex, obliquely ovate, anterior side abruptly truncate, narrow towards the 

 hinge border, posterior border rounded, auricles small, nearly equal; costae 25 28, 

 angulated, elevated, the angle being crested with a very fine carina, interstitial spaces 

 wide, conformable, each having a single very fine costa, the costae. and their carinae are 

 finely and densely striated transversely. 



The costae and sulcations are large upon the middle of the shell and become regularly 

 smaller towards the sides, becoming ultimately only so many fine lines. It is distin- 

 guished from Limea diiplicata, an Inferior Oolite species, by the more oblique form and 

 less elevated and acute costae. The Lima alternicosta of Buvignier, from the Ferruginous 

 Oolite, in the Oxfordian strata of the Department of the Meuse, does not appear to differ 

 from our species in any essential character, except that the figure he has given is 

 somewhat more than usually oblique. 



It is one of the most common bivalves in the formation, but it is not often that 

 the fine longitudinal carina upon the costae is preserved. Height, 14 lines ; length, 9 lines. 



Localities. The entire Minchinhampton district of the Great Oolite, also in the 

 Bradford Clay, Forest Marble, and Cornbrash of Wiltshire and Gloucestershire. 



(c.) LIMA PECTINIFORMIS, Schloth. Tab. VI, fig. 9. 



OSTRACITES PECTINIFORMIS, Schloth. Petref., p. 231, 1820. 

 OSTREA PECTIMFORMIS, Zieten. Wurtt., p. 62, t. 47, f. 1. 

 LIMA PROBOSCIDEA, Sow. Min. Con., t. 264, 1821. 



Golf/fuss. Petref., p. 8#/t. 103, f. 2. 

 PECTINIFORMIS, Bronn. LetU. Gcog., p. 214, t. 19, f. 9, 10, 1851. 



