4 ANNALS NEW YORK ACADEMY OF SCIENCES 



All modifications aud variations in progressive series tend to appear first in 

 the adolescent or adult stages of growth, and then to be inherited in successive 

 descendants at earlier and earlier stages according to the law of acceleration, 

 until they either become embryonic, or are crowded out of the organization, 

 and replaced in the development by characteristics of later origin. 



This law is well illustrated in the development of two Paris Basin 

 shells, Potamidopsis tricarinata Lamarck of the Calcaire Grossier and 

 Potamidopsis roissyi Desh. of the Sables Moyens. In the young, the 

 surface ornamentation of the latter species consists of two rows of nodes, 

 the lower being the more prominent. The same is true of the young P. 

 tricarinata, but this stage lasts much longer in the latter than in the 

 former species. The stage with two rows of nodes is followed by one in 

 which there is an additional ro\v of fine nodes intercalated between the 

 two, and the lowest row is still the most prominent. At about the ninth 

 volution of P. roisstji this ornamentation is fully developed, and it repre- 

 sents the adult ornamentation of P. tricarinata, which is found at a lower 

 geological horizon. The adult characteristics of P. tricarinata last for 

 hardly more than one volution on P. roissyi, which soon develops its own 

 characteristic adult ornamentation of three rows of nodes, the uppermost 

 of which are largest and transversely elongated. (See Deshayes, 1824, 

 plate L, fig. 13.) P. tricarinata is the ancestor from which P. roissyi 

 developed, and the latter records this fact in its ontogeny. 



Two individuals belonging to the same phyletic series and living dur- 

 ing the same time period frequently do not reach the same degree of 

 complexity in structure. One may retain its primitive characteristics 

 until late in its life history, only diverging from the ancestral type on 

 becoming fully adult, while another may pass through its ancestral stages 

 early in life and show a long succession of characteristics of later origin. 

 The former is retarded in its development in comparison with the latter. 

 Cerithium tuberosum is an illustration of a retarded shell, retaining, as 

 it does, the two equally strong spirals imtil the tenth volution, the whorls 

 only acquiring their acute angled outline on the twelfth volution. Ce- 

 rithium adansoni passes through the same ancestral stages as C. tuber- 

 osum, but loses the obtuse angled outline of the whorls at about the sixth 

 volution and after that acquires nodes, blunt spines and numerous addi- 

 tional spirals. It is a highly accelerated recent shell. 



Acceleration and retardation are expressed not only in the ontogeny 

 of the individual as a whole, but each character may be independently 

 accelerated or retarded. Cerithium graciliforme (Tryon, 1887, plate 

 xiii, fig. 77) retains the obtuse angled outline of the whorls to the 

 ninth volution, while C. echinatum (Kobelt, 1898, plate xx, fig. 6) loses 



