520 THE AMERICAN NATURALIST. [Vol. XXXVII. 



a seventh, next to the keel. Finally an eighth spiral appears on 

 the sutural side before the completion of the fifth conch volu- 

 tion. All these spirals soon become of nearly uniform size, while 

 a second cycle of spirals begins to appear between those of the 

 first cycle as they diverge with the increase in .width of the 

 shoulder. A supplementary spiral also appears just within the 

 sutural canal, next to the bounding ridge. Where these spirals 

 are crossed by the lines of growth, short, pointed, horny spines 

 are found on the periostracum. On the body of the shell inter- 

 calation begins much earlier than on the shoulder. In accel- 

 erated individuals intercalated spirals appear earlier on the 

 shoulder. 



Sycotypus pynts of the modern Floridian fauna seems to be a 

 more accelerated type. The smooth and ribbed stages are passed 

 through quickly, occupying only about half a volution. The 

 tubercles continue to the end of the second conch volution, after 

 which a smooth keel succeeds. When the shell has reached the 

 size where, in S. canaliculatus, the tubercles change into the 

 smooth keel (between the fourth and fifth volutions), the keel 

 in pynts begins to disappear, and the curvature of the outer 

 lip becomes uniform. In large (adult) individuals the carina has 

 generally disappeared altogether, and the lip is rounded uniformly. 

 This produces the "excavatus " type of lip. The canal begins in 

 the second volution of the conch. In two old Floridian shells of 

 -S'. pynts, in the collection of the American Museum of Natural 

 History, the canal becomes narrower and less pronounced in the 

 last whorl, and in some instances it disappears altogether for a 

 short space. In a characteristic specimen live spirals have devel- 

 oped by the end of the first conch volution. The sixth appears 

 immediately after the beginning of the second volution, and the 

 seventh begins in the third volution. Near the end of this volu- 

 tion intercalated spirals (of the second series) also appear, while 

 in S. canaliculatus these generally do not appear until the begin- 

 ning of the sixth volution. In this species the amount of embrac- 

 ing varies, thus changing the elevation of the spire. 



