1915.] 



NATURAL SCIENCES OF PHILADELPHIA. 



375 



The sculpture is less variable in this lot than in some others. 

 Ten specimens, taken at random, have 10, 12, 12, 13, 13, 13, 15, 17, 

 17, 17 ribs on the penultimate whorl. These fairly represent the 

 lot, so far as can be told without extensive counting. None counted 

 have more than 17 ribs (see PI. XIV, figs. 1, la, 16). 



Station 3 (summit of ridge above Station 2), but west of where 

 the trail crosses ridge). Shells exactly like those of Station 2, but 

 perhaps a little more variable in size, length 9 to 12^ mm. in extreme 

 specimens. 



Station 4 (summit of ridge further northwest, several hundred 

 feet higher than Station 3). These shells are conspicuously larger 

 than at Stations 2 and 3. Part of the shells are typical in sculpture, 

 but in most of them the ribs are much more numerous, closer, smooth, 

 and more regularly spaced. These close-ribbed shells agree with 

 those from Station 5 and from Station 12, a peak on the opposite 

 (east) side of the rim of the amphitheatre of Tweed Canyon. Proba- 

 bly the close-ribbed type of shell extends around the whole rim 

 from Station 4 to Station 12. 



Two out of fifteen opened have 3 lamellae (both having many ribs), 

 and three have only the columellar lamella (ribs few). The rest, 

 including both many- and few-ribbed shells, have 2 lamellae. This 

 lot was picked up in several places along the summit of the narrow 

 ridge, perhaps in an area of 20 x 100 yards. It therefore may com- 

 prise several colonies, and we cannot now tell whether fine- and 

 coarse-ribbed shells occur actually together or not. There may be 

 12-15 ribbed colonies and 20-30 ribbed colonies, or possibly both 

 sorts may live together. The measurements give extremes of size 

 and are from "selected" shells. 



Length 12.5, diam. 4.1 mm.; whorls 12f ; lamellae 3 



Station 5 (north of summit of peak north of Station 4), Shells are 

 like the fine-ribbed ones from Station 4. No really coarse-ribbed 

 forms were taken. Extreme and average shells measure as follows: 



