BIVALVIA. 37 



Gervillia tortuosa, Sow., sp. var. Tab. XL, fig. 25. 



Gastroch^na tortuosa, Sow. Min. Con., t. 52(5, fig. 1. 



— — Phil. Geol. York., t. 11, fig. ;56. 

 Gervillia tortuosa, Mor. Catal. Brit. Foss., 1854, p. 168. 



— — Oppel. Jurnformation, p. 418. 



Testa eIo»gata, antice tortuosa, postice recto, attenuato, umboni jjerobliquo, subter- 

 minali ; linea cardmis obliquissimo ; marline antico undulato, plicis concentricis Iceviter 

 instructis. Vaiva dextra subconcava, tortuosa ; fades interna ignota. 



Shell elongated, convex, and contorted anteriorly, straight and attenuated posteriorly ; 

 umbo very oblique, with a small anterior auricle; hinge-border very oblique, anterior 

 border undulated ; the surface of the valve has five irregular, concentric plications upon 

 its anterior side. The right valve is twisted conformably with the left valve ; it is some- 

 what concave. The cardinal ligamentary pits have not been exposed in the present variety. 

 Compared with the Inferior Oolite forms of Gervillia tortuosa, this variety is more 

 narrow and less contorted, having the posterior extremity nearly straight ; the umbo is 

 also more acute, and the surface is destitute of the large, rugose, concentric plications 

 which are conspicuous upon well-preserved examples of the typical form. So much 

 variability, however, is seen in the contorted species of Gervillia that I prefer to regard 

 the present as pertaining to G. tortuosa, but constituting a well-marked variety. 



Geological Position and Locality. It occurs rarely in the Cornbrash of Scarborough ; 

 from the collection of Mr. Leckenby. 



Gervillia Islipensis, Lye. Tab. XL, fig. 35. 



Testa, valva sinistra crassa, obliqua, convexa, linea cardinis elongata, auricula postica 

 permagna, falciformi, dorso angulo ohtuso obliquo instructo ; plicis incrementi paucis. Valva 

 altera etfoveolis interni ignotis. 



Shell with the left valve thick, inflated, very oblique, with a lengthened hinge-line and 

 falciform posterior auricle ; the anterior border is slightly excavated, and there is an 

 obtuse, oblique angle, which extends from the umbo to the posterior extremity of the 

 valve, which is curved backwards ; the folds of growth are few and prominent ; the 

 surface is destitute of ornament. The other valve is not known, nor has the hinge been 

 exposed. 



The general figure much resembles that of G. crassicosta, Mor. and Lye, but it is 

 more inflated ; it has a greater posterior curvature, and is destitute of the oblique costse. 

 The length of our largest example is 2 inches, that of the hinge-line I5 inch. 



Geological Position and Locality. The Cornbrash of Islip, Oxon., also the Great 

 Oolite of Stonesfield ; collected by Mr. Wliiteaves. 



