96 PROCEEDINGS OF THE ACADEMY OF [Feb., 
unite some of them specifically,?! are wholly due to evolution along 
parallel lines in the two stocks. This orthogenetic 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 V-shaped parietal tooth, like that of Polygyra. 
While the shell is extremely conservative in Sonorella, 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. varicifera 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 
Cle COTES Hae see ica te cos) wesc apna ce ee A. chiricahuana. 
a'!.—Shell light brown without much luster; usually with a callous 
ledge within the outer lip, or other weak vestiges of teeth. 
A. esuritor. 
b.—Penis normal. 
b!.—No penis; vagina long, much swollen above..A. metamorphosa. 
I1.—Aperture obstructed by three or four large teeth. 
a.—Very acutely carinate, the carina projecting above the suture; 
SVANOFIS OL CG SMES la orc c2css<-aestvaceecest eee oat eee A. jferrisst. 
a!,—Carina not projecting at sutures or wanting. 
b.—Surface papillose or scaly; shell thin, strongly carinate. 
A. lepiderma. 
b'.—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 aslight lip-rib only.............. A. duplicidens. 
d'.—Periphery strongly angular or carinate. 
21 In 1905 we considered the Chiricahuan Ashmunella proxima to be a sub- 
species of the Huachucan A. levettei, The Huachucan A. varictfera we thought 
identical with the Chiricahuan A. chiricahuana. 
r 
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