390 



NATURAL SCIENCE. Dec. 



however, removed after reading the "Challenger" Report on the 

 Lamellibranchiata (vol. xiii., p. 5), in which Mr. E. A. Smith, referring 

 to deep-sea forms, says : " We might multiply examples of the 

 different ranges in depth at which various species have been obtained 

 by the ' Challenger ' and other expeditions, but those which have 

 been cited are sufficient to show that the same species is equally 

 well adapted for living in deep or shallow water, and, as far as our 

 observations have reached, the shells appear to be very little affected by 

 the difference of the depth or the nature of the bottom." This 

 opinion was all the more gratifying as it fully confirmed the con- 

 clusions I had based on my own studies of the Cretaceous faunal 

 distribution. 



Attention must now be drawn to two exceptions to the rule here 

 laid down, which demand more extended consideration. Lithological 

 evidence points to the close of the Middle Chalk period as having 

 been a time of change, probably in the direction of reelevation. The 

 palaeontological facts seem peculiarly striking in this respect. Holaster, 

 of which H. subglobostts was the highest of the Grey Chalk Marl 

 species, now reappears in the Chalk Rock as Holaster planus. Ammo- 

 nites and scaphites, which were last met with in abundance in the Grey 

 Chalk, return in the Chalk Rock of Oxfordshire as the species Pachy- 

 discus prosperianus and Scaphites geinitzi. The arenaceous Foraminifera, 

 the abundant quartz grains from the residues, the crystals of tourma- 

 line, were all left behind in the Grey Chalk, but again we find them 

 appearing in the Chalk Rock. Is there no lesson, no significance in 

 this ? To me it seems clearly to indicate that the distribution of the 

 animal forms is in the main independent of the character of the sea- 

 bottom, and is far more influenced by temperature as a direct result 

 of depth, and by the strength of the prevailing currents. Here, then, 

 we see the principle of colonies supported by historical data. Time 

 has produced some alteration in the species, but genera reappear 

 which apparently had disappeared from the stage of Cretaceous 

 history, these belonging to the most varied groups, and identical with 

 those that existed throughout the whole of the Lower Chalk period. If 

 this be so with the Cephalopoda, should we not expect that, in some 

 measure at least, the other Mollusca would also be affected ? Here, 

 too, the results are of a character to convince the most sceptical. T^ivbo 

 gemmatus, and forms of TvocJms and Solayium, again resume their place 

 among the important fossils of this series. The trochoid forms are 

 the last to disappear during depression, and they are also the first lo 

 reappear during elevation, and should my theory be accepted as 

 having a firm and logical basis, it would no longer be necessary to go 

 to lithological characters to learn whether elevation or depression had 

 taken place, since the fossils themselves would clearly establish the 

 physical changes of the period. 



At the close of the Upper Cretaceous period, a remarkable group — 

 rare, it is true, but preserved in our great national collection — claims 



