348 PROCEEDINGS OF THE ACADEMY OF [May, 



shoulder angle, where sometimes all trace of the rib has disappeared, 

 when below the angle the rib is still well marked. As a rule, when the 

 tubercles become very high and sharp (spines) the ribs practically 

 disappear. 



When fine spirals cover the early whorls below the shoulder angle, 

 they are apt to disappear near the shoulder angle first in the later 

 adult whorls. As the shell grows, these fine spirals are found in a more 

 and more anterior position, until in old age only a few occur on the 

 branchial siphon. 



This briefly simis up the more important changes which are passed 

 through in the development of most individuals of Volutilithes petr-osus, 

 and many other species of the genus. Many individuals and some races 

 of Volutilithes petrosus pass through further stages of development; 

 but these are all of a gerontic character, and may be now more fittiniily 

 discussed under the following section. 



Senility. 



In widely separated genera of gastropods the writer has observed 

 that similar features are frequently found in the last whorl or in the 

 last few whorls. These features are never followed, in any gastropod, 

 by other and different ones, but are the last ornamental characters 

 which the shell acquires. They are the marks of old age or senility. 

 Many species and even genera and families never exliibit them, for in 

 the Palaeozoic they are rare and in the Mesozoic they are uncommon. 

 In Tertiary and Recent time, however, few large families of gastropods 

 are lacking in senile representatives. These representatives may be 

 genera, species or individuals. 



Individual Senility. — In many species, the vast majority of individ- 

 uals die at the period of mature shell ornamentation. Senile characters 

 are found only in an occasional specimen, which, as a rule, is either a 

 very large old one or else a dwarfed or pathologic individual.^ In indi- 

 vidual senility, the senile features are seldom accelerated to any extent 

 — that is, they are, as a rule, found in the last whorl, or in the latter 

 part of the last whorl. 



Specific Senility. — Often all the individuals of a species will show 

 signs of senility in the last whorl, or in the last few whorls. This 

 senile ornamentation of the species is more apt to be accelerated than 

 the senile ornamentation of the individual ; or, in other words, it occurs 

 earlier in the growth of the shell, 



® A similar condition has been noted by Hyatt for the Cephalopoda: " Biological 

 Relations of the Jurassic Ammonites," Proc. Bost. Soc. Nat. Hist., Vol. XVII, 

 1874, p. 239. 



