14 HYATT ON THE TERTIARY SPECIES 



the more flattened and more involute whorls of the full grown PI. ™Sf% but retain 

 throughout life with very sHght changes, the cylindrical form of the very youngest 

 stages of that shell. Like the imcoiling, it indicates the weakness of the animals, which 

 fail in the power of growth and cannot continue even the normal rate of increase 

 in the size of the shell which distinguished them in their later stages, and adult condi- 

 tion. 



It is very evident, however, that all of these retrogressive characteristics cannot be con- 

 sidered as pathological in exactly the same sense as the results of individual cases of 

 disease among the progressive species. They here affect three entire series of forms which 

 exhibit their impaired natural powers in various degrees, in one series as has been 

 shown, mingled with the advent of new characteristics, and in another, the third sub- 

 series, so completely subservient to these new characteristics, that the forms become 

 representative, notwithstanding their derivation from PI. '"fej^f* and decrease in size, with 

 those of the progressive series. 



The gradations and the numbers, and perfectly normal aspect of these shells as regards 

 their thickness, external markings and so on, as well as the increase in size noted in first 

 sub-series show that we must look to some cause which has affected their entire develop- 

 ment and lessened their powers of growth, finally leading through heredity to evidently 

 normal and general distortion. The retrogressive sub-series of the First Series, furnish 

 therefore, a very sharp contrast with the picture presented as a whole by the purely 

 progressive series. 



In the Second, Third and Fourth Series, there is a purely progressive tendency towards 

 increase in involution, in size, in spirality, and in the addition of new characteristics. In 

 the retrogressive sub-series on the other hand, there is a progression in some respects, 

 and a retrogression in others. 



The progressive characteristics are, however, but feebly manifested. Thus the decrease 

 in size from PI. levis to PI. minutus, fig. 18, is the most marked characteristic, and after 

 that the increase in size is confined to the normal members of the costatus group, the finely 

 costate and the coarsely costate shells, which are similar to fig. 26, and lines h and k, pi. 4. 

 The distorted forms corresponding to these, figs. 22-24, 28, pi. 9, and lines d, e, f, g, k, pi. 

 4, are nearly all small, and these are more numerous in the first or costate Sub-series, than 

 the larger shells. In the third sub-series no increase in size can be truthfully predicated, as 

 may be seen on pi. 4, lines a, b, c, though in these forms as has been stated, there are 

 carinations and sulcations, and slightly turbinate forms produced, which are decidedly 

 progressive in these characteristics. These facts seem to show clearly that each sub-series 

 has a history of its own in which both progressive and retrogressive tendencies are active 

 in different degrees. 



The tendency then to produce forms steadily increasing in spirality, is the only progres- 

 sive characteristic common to all the series shown in the table, and is evidently a prepo- 

 tent characteristic of all the Steinheim species, as it is in fact of most of the divisions of 

 the shell-bearing Lamellibranchiata, Gasteropoda, and Cephalopoda. 



Eliminating this characteristic and laying it aside for future consideration, let us now 

 turn to the very evident selection which has been exercised between the retrogressive and 

 progressive characteristics of the different series and sub-series. It has been shown, that 



