Vol. 59.] A LOWEK-GREENSANU FOSSILIFEROUS BAND. 261 



indentation at the front-margin, caused by the apex of the central 

 fold of the smaller valve : on each side of the central wide fold 

 is a narrow fold. Beak short, recurved ; foramen of a moderate 

 size : beak-ridges well defined, leaving a flattened area between 

 them and the hinge- line ; the smaller valve is divided by a fold 

 into three nearly equal parts. 



Dimensions: — length = 11 millimetres; breadth = 17 mm.; 

 thickness = 8 mm. 



Rhtnchonella leightonensis, nobis. (PL XVIII, figs. Ha-Scl.) 



Shell triangular in outline, globose, length and breadth nearly 

 equal, strongly compressed laterally towards the beak ; the greatest 

 thickness is about one-third distant from the posterior end ; the 

 greatest breadth is at nearly the centre of the shell; each side of the 

 front-margin rises by a gradual curve to about a third of the width 

 of the shell, from which it is nearly straight, forming a central wide 

 fold. The shell is covered by a multitude of fine, well-developed 

 plaits, which extend to the extremities of the beak on both valves. 

 The larger valve has its greatest width near the centre, from which 

 a broad deep central sulcus extends to the front-margin, forming a 

 wide fold ; the valve slopes with a slight curve towards the beak, 

 which is short and moderately incurved ; the beak ridges are 

 rounded ; there is a wide pseudo-area ; the foramen is moderately 

 large, and bounded by the deltidial plates. The smaller valve is 

 very globose, having its greatest thickness about the middle ; the 

 beak of the smaller valve is broad and tumid, and is compressed 

 towards the posterior part of the lateral margin. There is no 

 elevation corresponding to the deep sulcus of the larger valve by 

 which the centre of the front-margin is widely indented. 



Dimensions : — length =19 millimetres; breadth = 18*5 mm. ; 

 thickness = 13*5 mm. ; and the apical angle = 95°. 



By its peculiar form this species is easily distinguished from 

 other Cretaceous species. We have obtained recently several young 

 specimens of it. 



Khynchonella dimidiata, Sow. 



We have several young shells, but have not obtained a sufficient 

 number of adult specimens to investigate properly this species. 



Ehynchonella latissima, Sow. 



Some specimens may belong to this species. 



Khynchonella antidichotoma, Buv. 



We have found only one perfect Rhynchonella which may belong 

 to this species, and a very broad specimen which may be a variety 

 of it. 



We have many Terebratulce, often represented by single specimens, 

 which we are unable, for want of material, to assign to their several 

 species. Some of these are probably new forms. 



