1905.] 



NATURAL SCIENCES OF PHILADELPHIA. 



357 



Senile offshoots, 



representing rapid evolution 



(short Uved) . 



Adventitious Senility. 



An acceleration of shell characters, produced by accident or by patho- 

 logic conditions, occm-s in species of widely separated groups of gastro- 

 pods. Hyatt^^ observed that " a wound and its results, whatever they 

 may be, .... is primarily a severe shock to the system which lays 

 additional biudens upon the powers of grow^th, and is usually followed, 

 if severe, by retrogressive metamorphoses, or premature old age." 



Individuals of Volutilithes petrosus whose shells in the later whorls 



""The Genesis of the Tertiary Species of Planorbis at Steinheim," p. 15, 

 Anniversary Memoirs of the Boston Society of Natural History, 1880. 



