THE CAMBRIDGE GAXJLT AND GREENSAND. 295 



presence of a second septum directed upwards ; but more specimens 

 must be found before it can be proved that the indentation on the 

 cast is not the result of an accidental growth. Its unsymmetrical 

 position makes this at least possible. 



Dentalittm: decussattjm, Sow. 



Dentalium decussation, Sow. Min. Conch, pi. 70. f. 5. 

 Dentalium elli/pticum, Sow. ibid. pi. 70. f. 6. 



The casts of these two forms, so well known from the Folkestone 

 Gault, are perfectly un distinguishable ; but the shell of D. elli/pticum 

 is thicker across one diameter than the other, thus giving the section 

 an elliptical shape. Mr. Price considers, however, that this is 

 merely a varietal difference ; and Mr. Etheridge is inclined to agree 

 with him. It is one of the commonest fossils at Cambridge, oc- 

 curring everywhere, but always in the condition of black casts. 



Lamellibranchiata. 

 Ostrea erons, Park. 



Ostrea frons, Park. Org. Bern. pi. xv. f. 4. 

 Ostrea carinata, Sow. (pars), Min. Conch, pi. 365. f. 1-3. 

 Ostrea Milletiana, D'Orb. Pal. Fr. iii. pi. 472. f. 5-7; Pict. & Camp. 

 Ste.-Croix, p. 309, pi. 194. f. 7-9. 



This fossil ranges from the Upper Gault at Folkestone, where it 

 has recently been found, to the Grey Chalk of the same locality. It 

 is common in the Cambridge bed, some specimens being much water- 

 worn and containing a phosphatic cast, while others are well pre- 

 served and only filled with sandy marl, in which small phosphatic 

 nodules are sometimes imbedded. Their condition, therefore, is per- 

 fectly consistent with their range, some having been derived, and 

 others having lived during the deposition of the bed. M. Eenevier 

 finds it in the "Gault superieur" of Cheville, and remarks that some 

 of his specimens from Cambridge show a slight transition towards the 

 true 0. carinata ; these forms have, I believe, gOne under the name 

 of 0. macrojptera at Cambridge. MM. Pictet and Campiche discuss 

 the relations of 0. Milletiana at p. 310 of their monograph. 



Ostrea vesicttlaris, Lam. 



Ostrea vesicularis, D'Orb. Pal. Fr. p. 742, pi. 487. 

 Chama canaliculata, Sow. Min. Conch, pi. 26. f. 1. 

 Ostrea canaliculata, D'Orb. Pal. Fr. p. 709, pi. 471. f. 4-8 : Pict. 

 & Camp. Ste.-Croix, p. 305, pi. 193. f. 4-14. 



Mr. Etheridge agrees with me in thinking that the same shell has 

 been described under two different names according to the formation 

 in which it was found ; it seems generally to have been called O. 

 canaliculata when obtained from the Gault (see D'Orbigny, loc. cit.), 

 and O. vesicularis when found in Greensand and Chalk. 



A comparison of the figures of the two species above cited will 

 disclose how little difference there is between them ; and a study of 

 the fossils themselves results in finding the same series of forms in 



