Hyatt.] 378 [June 7, 



The size of the individual continues to increase during life, there is 

 no distortion, and only a normal tendency to the suppression of the 

 ribs. So far as the series, however, is concerned, the size of the later 

 occurring species or normal senile forms is smaller than that of the 

 average of the ancestral forms. 



The senile forms of this series and of the coronalum series express 

 in the continuous increase of the individual by growth throughout 

 life, and in the absence of all decrease in the amount of involution, a 

 certain power to resist the retrogressive changes which are so marked 

 in other series. The suppression of the tubercles, however, and the 

 narrowing of the abdomen and decrease in absolute size of the term- 

 inal species of the series are decisively senile. There is evidently a 

 mingling of opposing tendencies in these forms not found in the senile 

 forms of series, which are more completely changed. Thus in the 

 series from Brocclrii to Brongniartii, there is not only a retrogression 

 in absolute size, but also in the increasing distortion of the living 

 chamber. The period at which the living chamber begins to show 

 a distorted form and smooth exterior, becomes earlier in each spe- 

 cies. This is also true of the series leading into microstomum and 

 platystomum, which present similar characteristics. 



In dimorphum, however, which appears to be one of this group, a 

 remarkable difference makes its appearance. The living chamber is 

 no longer fully absorbed after each period of arrest in the growth, 

 and an abdominal channel, which was only occasionally visible in 

 some of the planulum forms, becomes quite constant. Nothwithstand- 

 ing these new characteristics, the form is evidently retrograde and 

 senile, suffering in some individuals from a very rapid series of senile 

 degradations. This is probably due to the declining force, which 

 prevents the animal from resorbing the walls of the living chamber. 

 A similar state of affairs occurs in the Sauzei series, where a new 

 characteristic is added in the shape of mouth lappets, but the inher- 

 itance of the distorted form of the living chamber takes place, as in 

 the Brongniartiian series. 



Every one of these series presents three principal stages of 

 growth and development, the young or Pettos-like, the adult, or that 

 in which Humphriesianum-like ribs and tubercles and a rounded ab- 

 domen appear, and an old age or senile period, in which these orna- 

 ments tend to disappear, the shell to decrease in size, and so on. 

 These three stages are present in different proportions in different 

 series. Thus the manifestations of a retrogressive tendency in Bay- 



