THE CAMBRIDGE GAULT AND GREENSAND. 283 



same time imparting to them a peculiar character which would not 

 be observable elsewhere. 



This succeeding formation was the Chalk-marl; and hence in 

 Cambridgeshire its lower strata exhibit so pebbly and sandy a con- 

 stitution as to have caused them to be referred to the Upper Green- 

 sand. Now, however, both stratigraphical and palseontological evi- 

 dence unite in showing this supposition to be erroneous ; and I sub- 

 mit that even the lowest of these beds cannot any longer be called 

 Upper Greensand. Stratic/raphically it is Chalk-marl, and should be 

 mapped and spoken of as such ; but palceontologically considered, 

 what is usually regarded as the characteristic part of its fauna un- 

 doubtedly belonged to that of the Upper Gault : and though it is 

 quite an exceptional state of affairs for the majority of fossils con- 

 tained in a bed to be of a different age from the stratum itself, still 

 such is the case here ; and I think, therefore, the species included in 

 the " derived fauna " of the appended list should certainly be ad- 

 mitted as Upper Gault forms, and hereafter be quoted as belonging 

 to that division. 



These palaeontological considerations bear upon another important 

 question, viz. the age of the Hunstanton limestone. Mr. Seeley 

 argues this to be Upper Greensand, chiefly on the ground of its con- 

 taining so many of the Cambridge fossils ; if, however, these belong 

 to the fauna of the Upper Gault, this argument changes its direction, 

 and points to the latter formation as one which the Hunstanton red 

 rock may in part represent. This explanation would at any rate 

 account for the mixture of species it contains, and also for the differ- 

 ent opinions entertained regarding its age, some geologists main- 

 taining it to be Gault, others as confidently calling it Upper Green- 

 sand. 



I hope, however, to make this the subject of a future communica- 

 tion to the Society, in which I shall endeavour to trace the relations 

 of the strata northward from Cambridge, as in this paper I have 

 followed a line south-west of that town. 



§4. Remarks on the Determination and Synonymy of certain Species. 

 — The following observations are the result of a long and careful study 

 of the Cambridge fossils ; and I have been led to identify several 

 hitherto unnamed species in consequence of having what I consider 

 the right clue to the origin of the fossils in their derivation from the 

 Gault. 



It may therefore be objected that I have interpreted the forms 

 so as to be in accordance with this preconceived theory . I am quite 

 willing to admit it, and to acknowledge that a belief in the Gault 

 origin of the fauna has led me also to doubt the accuracy of many 

 previous identifications, and to look for their true representatives 

 among the fossils of the Gault. At the same time I hold that there 

 is nothing unscientific in such a mode of procedure, and that, in the 

 present state of palaeontology, it is very desirable to have some such 

 guide amid the difficulties arising from the multiplication of species, 

 and from the comparison of specimens in different states of preser- 

 vation and mineralization. 



