1905.] NATURAL SCIENCES OF PHILADELPHIA. 235 



Monument Rock, Santa Fe Canyon, at 8,000 feet above the sea (PI. 

 13, fig. 29), is slightly larger with the umbilicus wider at the opening. 

 Alt. 7.2, diam. 14 mm.; whorls 5f. The basal tooth is simple. An- 

 other like it, taken by Ashmvm in the same canyon, is figured (PI. 13, 

 fig. 30). 



In all the forms referred to A. thomsoniana and its subspecies no dis- 

 tinct internal lip-rib is formed when a resting-stage occurred in the 

 neanic period. Such growth-arrests are often indicated by a darker 

 streak on the penultimate or beginning of the last whorl, but the shell 

 is very rarely thickened within by a white callous, and when present, 

 the callous is very thin. 



Distribution: Santa Fe Canyon (Thomson, Ashmun, CklL); Las 

 Vegas and Las Vegas Hot Springs (Miss Cooper, Prof. CklL); Canyon 

 Diablo, near Rowe (Mary Cooper) ; Old Pecos Pueblo, near Valle ranch, 

 Pecos (W. C. CklL). 



The form from Las Vegas Hot Springs, at about 7,000 feet elevation, 

 called A. t. cooperce, offers no tangible difference from Santa Fe thom- 

 soniana. It is not yet evolved enough to be distinguishable from 

 thomsoniana if the specimens were mixed. I do not think it desirable 

 or practicable to name forms so very slightly differentiated. The 

 umbilicus is a trifle more open than typical A. thomsoniana, like the 

 Monument Rock shells or a little less open, being thus intermediate in 

 size. The teeth are in the average slightly smaller. The basal tooth 

 is quite feebly doubled in a minority of the shells seen, simple, small 

 and tubercular in the others. The spire is, in the average, a trifle 

 higher than in Santa Fe thomsoniana (PL XIII, figs. 31-34, the last a 

 co-type of A. t. cooperce). 



A. antiqua CklL and Coop., from the Pleistocene at Las Vegas, New 

 Mexico, is like the shells from Las Vegas Hot Springs noted above in 

 size, but the basal tooth is a low, wide callous, slightly emarginate but 

 not distinctly bifid. The rather heavy teeth and small umbilicus are 

 like typical thomsoniana. A co-type measures 13.2 mm. diam. The 

 parietal callous has scaled off, carrying the tooth with it. I see no 

 valid reason for believing that it represents a divergent branch of 

 thomsoniana. Such characters as this are merely individual. 



