No. 440.] STUDIES OF GASTROPODA. 



the whorls embrace below the middle. The sutural canal 

 remains narrow, however, and the shell has more nearly the 

 aspect of S. canaliculatits. Intermediate types connect this with 

 S. excavatus and S. canaliferus. 



The ancestral type of these branches of Sycotypus is prob- 

 ably close to 5. rugosus (Conrad) in most characters (Fig. 9). 

 This Mid-Miocene species retains its tubercles 



throughout, though these become very promi- 



in the later whorls. The channel already 

 begins in the late third or early fourth whorl, 

 hence this is no new character. The number 

 of spirals is simple throughout, as far as 

 observed, but there are seven of them fully 

 formed in the fourth or fifth whorl. In some 

 specimens the shoulder tubercles begin to 



In 5. coronatus we have a form which com- 

 pares in the character of the sutural canal with the ancestral 

 type, but which has the tubercles strongly developed and the 

 spirals intercalated at an early stage, show ing acceleration in 

 these points. Another terminal type, S. cominuus, Conr. has 

 simpler spirals and a coronatus-like shoulder angle, which is 

 however more compressed and projects more than in that 

 species. 



The ancestral type of these species of Sycotypus is probably 

 to be sought in the Upper Oligocene mutations named by Dall. 

 tampacnsis and pcrizomitus. and referred as varieties to Conrad's 

 Fusus spinigcr. In the former of these, seven simple spirals 

 occur on the shoulder and a smooth space between the suture 

 and the first spiral in front of it. In the latter, intercalation of 

 secondary spirals begins on the shoulder after the fifth simple 

 one has appeared, and the smooth space next to the suture 

 becomes depressed into a eanaliculation. The mutation bnmsii 

 Dall of the Upper Oligocene, seems to be still more primitive in 



