96 PROCEEDINGS OF THE ACADEMY OF [Feb., 



unite some of them specifically, 21 are wholly due to evolution along 

 parallel lines in the two stocks. This orthogenet ic evolution has 

 resulted in the most extraordinary parallelism. By degeneration 

 of the teeth of the aperture, wholly toothless forms have arisen 

 as terminal evolution products in both phyla; and these are so similar 

 in shell-characters that they are indistinguishable. Other forms in 

 both phyla have evolved a Y-shaped parietal tooth, like that of Polygyra. 

 While the shell is extremely conservative in Sonorclla, in the related 

 genus Ashmunella it appears to be the most plastic part of the 

 organism. As in Polygyra and many other molluscan groups in which 

 complex hard parts have been evolved, many races and species are 

 characterized by varying degrees of degeneration of the teeth of the 

 aperture. In the current phraseology, this may be due to the action 

 of an inhibiting factor, earlier or later checking the development of 

 teeth. This seems to have taken place independently in most of the 

 centres of Ashmunella evolution. Toothless apertures are no doubt 

 primitive in the Belogenous Helices, so that secondarily simplified 

 forms, such as A. varicijera and A. chiricahuana, have completed an 

 evolution cycle, returning to the primitive simplicity of aperture. 

 Whether such forms can give rise to new series having dentate 

 apertures remains uncertain. 



Key to Chiricahuan Ashmunellas. 



I. — Aperture toothless or with very small vestiges of teeth only. 

 a. — Shell chestnut brown, glossy, the lip narrow, without traces 



of teeth A. chiricahuana. 



a 1 . — Shell light brown without much luster; usually with a callous 

 ledge within the outer lip, or other weak vestiges of teeth. 



A. esuritor. 

 &.— Penis normal. 



6 1 , — No penis ; vagina long, much swollen above. A. metamorphosa. 

 II. — Aperture obstructed by three or four large teeth. 



a. — Very acutely carinate, the carina projecting above the suture; 



whorls of the spire flat A. ferrissi. 



a 1 . — Carina not projecting at sutures or wanting. 



b. — Surface papillose or scaly; shell thin, strongly carinate. 



A. lepiderma. 

 b 1 . — Surface not scaly or conspicuously papillose. 



c. — Basal tooth single, bifid, or with two basal teeth nearer 

 together than the outer one is to the outer lip tooth. 

 d. — Periphery obtusely angular in front, becoming rounded; 



young with a slight lip-rib only A . duplicidens. 



d 1 . — Periphery strongly angular or carinate. 



^ 21 In 1905 we considered the Chiricahuan Ashmunella proxima to be a sub- 

 species of the Huachucan ..4. levettei. The Huachucan A. varicifera we thought 

 identical with the Chiricahuan A. chiricahuana. 



