THE RECAPITULATION THEORY 295 



passes through the series of stages by which the 

 cyclical or discoidal type of shell was derived from 

 the simpler spiral form. 



In Orbitolites tenmsstma, as Dr. Carpenter has 

 shown,* "the whole transition is actually presented 

 during the successive stages of its growth. For it 

 begins life as a Cornuspira, .... its shell forming 

 a continuous spiral tube, with slight interruptions 

 at the points at which its successive extensions 

 commence ; while its sarcodic body consists of a 

 continuous coil with slight constrictions at intervals. 

 The second stage consists in the opening out of its 

 spire, and the division of its cavity at regular 

 intervals by transverse septa, traversed by separate 

 pores, exactly as in Peneroplis. The third stage is 

 marked by the subdivision of the ' peneropline ' 

 chambers into chamberlets, as in the early forms of 

 Orbiculina. And the fourth consists in the exchange 

 of the spiral for the cyclical plan of growth, which 

 is characteristic of Orbitolites ; a circular disc of 

 progressively increasing diameter being formed by 

 the addition of successive annular zones around the 

 entire periphery." 



The shells both of Foraminifera and of Mollusca 

 afford peculiarly instructive examples for the study 

 of recapitulation. As growth of the shell is effected 

 by the addition of new shelly matter to the part 

 already existing, the older parts of the shell are 

 retained, often unaltered, in the adult ; and in 

 favourable cases, as in Orbitolites tenuissima, all the 



* W. B. Carpenter, "On an Abyssal Type of the Genus Or- 

 bitolites," Phil. Trans. 1883, part ii. p. 553. 



