72 SECTIONAL ADDRESSES 



principle as that used in celluloid dolls with leaden bases. Once more 

 the shell stands upright, and all seems well. But the tendency to secrete 

 great quantities of carbonate of lime becomes, as Lang has shown in the 

 case of Polyzoa, an obsession. A full-grown G. incurva, when opened, 

 shows very little accommodation for the oyster in contrast to the bulk and 

 massiveness of the shell. With progressive increase in solidity, and 

 continuous further curvature of the once fixed valve, the shell becomes 

 unwieldy, and that particular lineage of oysters disappears. 



The tragic story of Gryphcea is in no sense unique, nor is it of a kind 

 peculiar to Mollusca. An exactly parallel case, or series of cases, can be 

 found among the Brachiopoda. The story of Productus could be told in 

 almost the same words as that of Gryphcea ; while there is hardly a family 

 of Brachiopoda that does not include some types in which undue curvature 

 of the ventral valve has obliterated the pedicle. Whatever ingenious 

 devices may have been employed to compensate for this condition, by 

 local weighting or spinous growths or coral-like cementation, there was 

 no long future in store for a Brachiopod stock that dissipated its birth- 

 right by destroying its pedicle. 



We have, of course, no direct evidence as to the changes, if any, that 

 occurred in the soft structures of oysters and Brachiopods whose shells 

 underwent such alterations. Probably some readjustment would be 

 needed to fit the change of posture, but it need not have been drastic. It 

 is, however, evident that the utmost perfection and efficiency of all the 

 other organs would avail nothing if and when the shell became unmanage- 

 able. Heart-failure will kill an otherwise healthy body. 



The account of morphogenetic evolution can be extended and amplified 

 by a further ' Analysis of the Genus Micraster.' Rowe's work on this 

 type, although not the first of its kind, deserves to be regarded as a classic 

 in evolutional studies. Perhaps its chief value lies in its avowedly strati- 

 graphical aim ; the chronological succession was recorded without regard 

 to any possible biological implications. Moreover, an Echinoid is an 

 exceptionally satisfactory type for palaeontological study. Its hard parts 

 include a great variety of structures, and the mesodermal and yet peri- 

 pheral character of its test ensures intimate association with the living 

 tissues and close contact with the environment. For our present purpose 

 it will suffice to select three distinct structures of the test, and to consider 

 only the simplest aspect of their progressive modification during the 

 period of the Upper Chalk. 



The interporiferous tract of the ambulacral petals is almost smooth in 

 such types as M. corbovis and M. leskei, species from the base of the Upper 

 Chalk. In the highest zones to which the genus persists, these tracts are 

 highly ornamented with granules, and marked by a pronounced groove 

 along the median line. Every gradation between these two extremes can 

 be found ; and, in spite of occasional slight irregularities, the chrono- 

 logical sequence of the gradations is remarkably straightforward. It is 

 not easy to suggest any functional difference of vital importance that this 

 change could indicate ; rather the steady increase in elaboration seems 



