300 A. J. JUKES-BROWNE ON THE RELATIONS OF 



over, the existence of two varieties, one shorter and more triangular 

 than the form figured by M. D'Orbigny ; both occur at Cambridge, 

 and are figured on PL XV. f. 4-6. 



Isoarca Agassizii, Pict. & Eoux. PI. XV. figs. 1-3. 



Isoarca Agassizii, Pict. & Eoux, Gres Verts, p. 446, pi. 38. f. 3. 



This shell is very rare at Cambridge ; but there are fortunately 

 five good specimens in the University collection, all of them casts 

 in dark phosphate, four of which agree closely with the figures 

 of MM. Pictet and Eoux. The fifth is a smaller variety, with less- 

 inflated umbones. Mr. Seeley has, I believe, given these fossils the 

 MS. name of I. cantahrigiensis ; but I see no reason for separating 

 them from the above species, especially as the shell of the Cambridge 

 specimens is not yet known. 



Ltjctna tenera, Sow. PI. XV. figs. 10-12. 



Venus tenera, Sow. Geol. Trans, ser. 2. vol. iv. pi. xi. f. 7. 



A single specimen of this shell is in the "Woodwardian Museum ; 

 and I have seen two other individuals. They are all black casts, 

 but well marked, and precisely similar to the casts and shells common 

 in one particular bed of the Folkestone Gault : this is Bed II. in the 

 Lower Gault of Mr. Price, and he tells me that the shell is not found 

 there at any other horizon. It is therefore characteristic of the 

 Lower Gault. 



CORBIS GAITLTINA, Pictet. PI. XV. fig. 9. 



Fimbria gaultina, Pictet ; Pict. & Camp. Ste.-Croix, iii. pi. 122. 

 f. 3, 4; Pict. & Eoux, Gres Verts, pi. 34. f. 4. 



There are two nearly perfect casts in the "Woodwardian Museum, 

 which I am inclined to refer to the above species. The undulated 

 growth of the shell leaves concentric markings upon the casts, which 

 closely resemble the figures mentioned above ; but I have not been 

 able to see the specimens themselves. The species is found in the 

 Alpine Gault and the Vraconnian of Ste.-Croix and Cheville. 



Pholadomya dectjssata, Phill. 



Cardium decussation, Fitton, Geol. Trans, vol. vi. p. 113. 



Pholadomya decussata, var. triangulata, Seeley, Ann. & Mag. Nat. 

 Hist. 1861, vol. vii. p. 122. 



These casts are rather dwarfed varieties of the Chalk form, but 

 agree with it in all essential characters ; they are seldom met with 

 near Cambridge, and are rarely in good preservation. The Cardium 

 decussatum of Sowerby, described originally from the Chalk-marl, 

 is cited by Dr. Fitton as having been also obtained from Copt Point 

 near Folkestone y so that the shell ranges from Gault to Chalk; 

 and the Cambridge specimens are clearly derived from the former 

 deposit, being waterworn casts in black phosphate. 



