1905.] NATURAL SCIENCES OF PHILADELPHIA. 349 



Generic Senility. — Here all the species of a genus will exhilDit signs 

 of senility, and the generic senility is more accelerated than the specific. 



Similarly, senility may extend to the genera of a family, and if, as 

 is probably the case, the VerinetidcB are senile, we have the characters 

 occurring early in the growth of shell, with the result that the forms 

 comprising the family differ in appearance to a marked degree from the 

 normal gastropod . 



The marks of senility may be conveniently grouped under several 

 heads. Two or three of them have been mentioned by Hyatt, Grabau 

 and other authors, and these are merely noted in the present paper for 

 the sake of completeness. The wi'iter knows of no case in which all 

 of the characters are found together. Frequently only one will indicate 

 that the individual, species or genus has passed its prime, but more 

 often two or more senile characters occur. As old age comes on the 

 marks of senility appear, but not necessarily all at the same time. 

 Usually one character will appear in the growth of the shell slightly 

 ahead of another character. Sometimes one of the senile characters 

 will be much accelerated and become prominent at a comparatively 

 early stage, while the others will be found on the last whorl only. 



The wTiter regards the following as marks of senility : 



1. Passing of the shoulder tubercles or spines into a shoulder keel. 



2. Swelling on the anterior siphon. 



3. Protrusion of the mantle in the region of the anal siphon, produc- 

 ing a smoothing of the preceding whorl by a shelly overgrowth. 



4. Encroachment of the anal siphon upward on the preceding whorl. 



5. Stromboid form to the outer lip of the aperture. 



6. Tendency toward loose coiling. 



7. Irregularity of growi:h lines. 



8. Thickening of the shell. 



9. Tendency to form a smooth and rounded whorl. 



10. Recurrent or discontinuous ornamentation. 



1. Passing of the shoulder tubercles or spines into o shoulder keel. — 

 This process is well shown in such forms as Fulgur carica and Voluti- 

 lithes petrosus, where little acceleration has occurred. The change 

 takes place by a shortening of the interval between spine formation, 

 with the result that the spines are nearer each other. The growth lines, 

 on which the spines are situated, are thus brought close together. 

 The spines become lower in height, but longer in a spiral direction, 

 until they fuse together into a continuous ridge, so that the final result 

 of the process is a shoulder keel without definite spines. 



In Fulgur canalicidatum acceleration has thrown the spines back 



