1905.] NATURAL SCIENCES OF PHILADELPHIA. 235 
Monument Rock, Santa Fé Canyon, at 8,000 feet above the sea CPI. 
13, fig. 29), is slightly larger with the wmbilicus wider at the opening. 
Alt. 7.2, diam. 14 mm.; whorls 53. The basal tooth is simple. An- 
other like it, taken by Ashmun in the same canyon, is figured CPI, 1s, 
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 Fé Canyon (Thomson, Ashmun, CkIl.); Las 
Vegas and Las Vegas Hot Springs (Miss Cooper, Prof. Ckll.); Canyon 
Diablo, near Rowe (Mary Cooper) ; Old Pecos Pueblo, near Vallé ranch, 
Pecos (W. C. CkIL.). 
The form from Las Vegas Hot Springs, at about 7,000 feet elevation, 
called A. t. coopere, offers no tangible difference from Santa Fé 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 secn, simple, small 
and tubercular in the others. The spire is, in the average, a trifle 
higher than in Santa Fé thomsoniana (Pl. XII, figs. 31-34, the last a 
co-type of A. t. coopere). 
Co-types of 
coopere. 
SS SSE 
Alt. 8 CO Noe! (69) ok ti 1.2 2) 4.) (6. mm, 
Diam. 13 its iss ets too Laan Ie. “ka 
Whorls 5+ 5+ 5+ 5% oF 5 54 5 43 
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: Lseetne 
valid reason for believing that it represents a divergent branch of 
thomsoniana. Such characters as this are merely individual. 
