460 M. O'Cpnnell — Ortho genetic Development. 



Skipping about a quarter of a volution, we may note the 

 costal arrangement on the sixth whorl from 6% to 6y 2 

 volutions (fig. 2). In the first triad in this inter sphinc- 

 teral sector the greatest strength has already passed to 

 the third costa, one is free and two free but still directed 

 toward three ; it corresponds approximately to the stage 

 of development shown in the second group of sector N. 

 being, however, a little more accelerated. The second 

 triad in X is about correlative with the third in N. The 

 third triad in X immediately precedes a constriction and 

 has certain special features which need not be discussed 

 here. Thus the beginning of each intersphincterial sec- 

 tor shows the costse at a more advanced stage of develop- 

 ment than they were at the beginning of the preceding 

 sector and the end of each sector is more advanced than 

 the end of the preceding one. The costse develop in a 

 definite way according to law but there is not uninter- 

 rupted progress throughout the life of the individual; 

 rather is there a constant repetition of what has been 

 done before by the animal and an addition of one step 

 more each time, so that the final adult stage shows a con- 

 dition far in advance of the youthful stage but the result 

 is obtained only by progress to a certain point, then an 

 interruption and a new beginning with progress again 

 a little beyond the point reached before the interruption, 

 and so on. 



This mode of costal development holds not only in this 

 species but in many others and the stages shown in the 

 single individual described are found to be characteristic 

 of the adults of earlier geologic representatives of the 

 genus, and in these the costse develop in the same way as 

 in the young P. cubanensis. Thus the definite direction 

 of development of the costse in the single specimen of 

 this species is not a matter of individual growth but is 

 some tendency inherent in the organism which leads to 

 the same type of development in related species and in 

 ancestors and descendants throughout Middle and Upper 

 Jurassic time. 



American Museum of Natural History, 

 New York City. 



