360 PROCEEDINGS OF THE ACADEMY OF [May, 



of succession may be the same as in forms whose evolutional rate is a 

 fast one. 



Lastly, an acceleration of characters may result from some accident 

 which occurs to an individual ; and in this case we may find a sudden 

 appearance of senile characters, which may occur in the normal onto- 

 geny of the species, at a later period, or they may be extra-specific ; that 

 is to say, never occurring in the ontogeny of the species, but found in 

 some senile offshoot from the same stock. 



The writer wishes to express his indebtedness to Prof. Amos P. Brown 

 of the University of Pennsylvania, to Prof. Henry A. Pilsbry, Mr. 

 Edward G. Vanatta and Mr. Witmer Stone of the Academy of Natural 

 Sciences of Philadelphia, and to Mr. C. W. Johnson of the Boston So- 

 ciety of Natural History, for many courtesies and kindly suggestions 

 and to Miss Helen Winchester for the care exercised in making the 

 drawings. 



Explanation of Plates XXX and XXXI. 



Plate XXX, fig. 1. — Volutilithes petrosus Con. Eocene. Mt. Lebanon, La. 

 Length = 16 mm. Young individual, showing the change from simple 

 curved ribs to straight tubercled ribs. In the last whorl the ribs 

 are well marked below the shoulder angle, but very faint above it. 



Fig. 2. — Apex of the above specimen. Length = 2 mm. Shows the change 

 from simple curved ribs to straight ribs with suture and shoulder 

 angle tubercles. 



Fig. 3. — Volutilithes petrosus Con. Eocene. Jackson, Miss. Diameter of 

 shell at last whorl measured on the largest spine = 24 mm. Apical 

 view of old individual with long spines. Here the tendency for the 

 spines to pass into a shoulder keel is only slightly marked, but they 

 decrease in size rapidly toward the aperture. The last whorl is 

 slightly smoothed by the protrusion of the mantle in the region of 

 the anal siphon. 



Fig. 4. — Volutilithes petrosus Con. Eocene. Bell's Landing, Ala. Diam- 

 eter of shell at last whorl = 33 mm. Apical view of old individual 

 exhibiting extreme senile characters, such as the passage of the 

 shoulder angle spines into a shoulder keel, encroachment of the anal 

 siphon upward, and the shelly overgrowth produced by the protrusion 

 of the mantle in the region of the anal siphon. 



Fig. 5. — A different view of the above individual. Length =.56 mm. Here 

 the senile characters mentioned imder Fig. 4 show up still more 

 plainly, especially the passage of the shoulder angle spines into a 

 shoulder keel. 



Fig_ 6. — Ecphora quadricostata Say. Miocene. St. Mary's River, Md. 

 Length =24 mm. Normal young individual. 



Fig. 7. — Ecphora quadricostata Say. Miocene. St. Mary's River, Md. 

 Length =23 mm. Example of adventitious senility. Young indi- 

 vidual which has been injured early in life and senile characters appear 

 early. Tendency toward loose coiling is well marked. 



Pig 8 — Ecphora quadricostata Say. Miocene. St. Mary's River, Md. 

 Length =76 mm. Normal adult individual exhibiting tendency 

 toward loose coiling. 



