MANUAL OF CONCHOLOGY. 



DESCRIPTIONS OP GENERA AND SPECIES. 

 Genus CHJENAXIS Pilsbry & Ferriss. 



Ch&naxis PILS. & FERR., Proc. A. N. S. Phila., 1906, p. 145. 



The shell is cylindric or slightly tapering, having a large, 

 hollow axis, open below, and about one-third the total diam- 

 eter of the shell; lamellae and plicae substantially as in the 

 typical group of Gastrocopta, the angular and parietal lamellae 

 being wholly concrescent into one lobed lamella; columellar 

 lamella well-developed, horizontal; a deeply entering supra- 

 columellar lamella is sometimes developed. Upper and lower 

 palatal plicae and basal plica present. 



Type C. tuba (P. & F.). The known species live in Arizona. 



A broadly open umbilicus at all stages of growth is un- 

 usual in Pupttlidcz. Its development in Chcenaxis seems suffi- 

 cient ground for generic rank, even though the rest of the 

 organization is essentially that of Gastrocopta, at least so far 

 as the shell is concerned. The position of the basal plica, as 

 well as the structure of the wholly concrescent angulo-parietal 

 lamella, show that Chcenaxis was derived from the southern 

 typical Gastrocopta stock, not from the northern Albinula 

 and Sinalbinula group of the genus. 



Key to Species. 



a. Last whorl having a long cord-like spiral lamella on the 

 axis. 



b. Internal lamella ~L~y 2 to 2 whorls long. 



C. intuscostata, no. 2. 

 b 1 . Internal lamella about 1 whorl long. 



C. i. brevicostata, no. 2a. 

 a 1 . Last whorl without a long spiral lamella inside. 



C. tuba, no. 1. 



