500 J. Siarlde Gardner — Relative Ages of 



age of considerable value. The absence of Ichthyosanrians, which 

 do not seem to have survived the Grey Chalk or Upper Greensand 

 period in England, is evidence of a negative kind, but not, on that 

 account, to be neglected. The replacement of Plesiosaurs by 

 Pliosaurs, and, in short, every fact connected with the distribution 

 of the few vertebrates that occur in our Cretaceous series, has a 

 significance if properly interpreted. The fish teach nothing as yet, 

 nor do the Crustacea. Many of the Echinodermata seem to have 

 remained stationary, while others require a more critical comparison 

 than they have yet received to determine in what direction, and 

 how far in it they have progressed. I fancy, however, that even a 

 cursory examination reveals a distinct progress in the Micrasters 

 from the Chalk of Limburg and Denmark, towards the Tertiary 

 types, and I entertain no doubt that an examination of the other 

 Echinoderms by a practised observer, such as Prof. P. M. Duncan, 

 would lead to important results. With this object in view, all the 

 minute organisms which combine to form the Chalk, together with 

 the Hexactinellida, stalked Crinoids and Brachiopods, require special 

 comparison with the species now living under similar conditions. 

 No Globigerina ooze of Tertiary age has yet been discovered. Most 

 of the genera of mollusca of the Chalk itself that were preserved are 

 now extinct, and we do not yet know in what direction they were 

 progressing, or even varying. As there is no reason, however, to 

 suppose that they all became extinct at once, but, on the contrary, 

 much evidence that they gradually dropt out, the percentage of 

 extinct types present in any formation would, to some extent, be an 

 index of its position in the Cretaceous record. As regards bivalve 

 mollusca, there does not seem to me to be any broad rule of 

 progression, and the tendency of individual genera must be traced. 

 The contrary appears the case with the Gasteropoda, if we set aside 

 the archaic helicoid, turbinate and patelloid groups and the tubular 

 Solenoconchia. We find a most unmistakable and pronounced ten- 

 dency to gradually elongate the canal; and the presence in greater 

 or less numbers, or absence, of fusiform shells with lengthened canals 

 would, I believe, be an infallible test of the age of any group of 

 Gasteropods from the Jurassics to the Eocene. The important Creta- 

 ceous family of Aporrhaidee, now confined to two species, reached its 

 zenith in the later Cretaceous rocks, and the presence of relatively 

 gigantic forms is, I feel convinced, a sure sign of a late deposit. 

 Moreover, in exactly the same ratio that the lessening proportion of 

 extinct genera indicates later and later stages, so does the incoming 

 of such distinctly new developments as cones and cowries. Where 

 any considerable group of mollusca is present, there should thus be 

 no great difficulty in determining the relative age of a deposit. No 

 kind of evidence, negative or positive, should be lost sight of how- 

 ever, nor should undue stress be laid upon any one group to the 

 exclusion of others ; but chiefly should specialists refrain from at- 

 tempting to decide complicated geological problems from their own 

 exclusive standpoints. 



If we examine the Upper Greensand of Aix-la-Chapelle with these 



