1905.] NATURAL SCIENCES OF PHILADELPHIA., 351 



development of certain featiu-es. The protrusion of the mantle onto 

 the preceding whorl is often accompanied by an encroachment of the 

 anal siphon upward onto the preceding whorl. 



There is evidently a close relation between the swelling on the 

 anterior siphon and the protrusion of the mantle on the preceding 

 whorl in the region of the anal siphon ; for as the latter feature becomes 

 stronger the former declines. The races of Volulilithes petrosus above 

 mentioned illustrate this fact perfectly. They range from forms with a 

 well-marked swelling to others with no swelling; and in one race with 

 an extreme mantle protrusion there is a depression in place of a swell- 

 ing. As the swelling has been diminishing, there has been a corre- 

 sponding increase in the amount of protrusion of the mantle on the 

 preceding whorl in the region of the anal siphon. 



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

 This often accompanies the preceding mark of senility and is evidently 

 produced by the same causes. It is well shown in Volutilithes petrosus 

 and in Fulgur carica, also in Strombus and Aporrhais, where it is accom- 

 panied by the wing-like flaring of the outer lip of the aperture. 



5. Stromhoid form to the outer lip of the aperture. — This is a flaring, 

 wing-like extension of the outer lip of the aperture and is found in 

 Strombus, Aporrhais and a few other forms. In Strombus it is often 

 accompanied by a thickening of the shell, by a bending of the branchial 

 siphon upward or backward, by the encroachment of the anal siphon 

 upward onto the preceding whorl, and by recurrent ornamentation 

 {Strombus leidyi). 



6. Tendency toward loose coiling. — In many gastropods the later 

 whorls are not closely applied against the preceding whorls. In other 

 words, the coiling becomes loose. Ecphora, Ropana and some species 

 of Murex do not show signs of uncoiling until well along in life. In 

 such forms as Vermetus, however, the process is further advanced, and 

 all trace of the original spiral condition is lost at an early date; the 

 shell becoming a mere irregular tube. 



7. Irregularity of growth lines. — In many gastropods {Volutilithes 

 petrosus is a good example) the final senile stages are characterized by 

 a thickening and irregularity of the fine lines of growth. Thi^ charac- 

 ter goes hand in hand with the thickening of the shell, the passing of 

 the spines into a shoulder keel, recm-rent ornamentation, and other 

 senile features. 



8. Thickening of the shell. — This is a character which occurs in the 

 senile stages of manj'- diverse groups of gastropods. It is usually ac- 

 companied by other senile features, and is probably more widely dis- 

 tributed than any other mark of senility. 



