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 Pl. XIV, figs. 1, la, 1b). 
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 123 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 38. 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 lamellz (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 20x 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 123; lamelle 3; ribs 21. 
ce a3 73 ce il ce a 86 
a 4 123: 9-6 OA! 
cc PAs cc 4 “ce ce 123; cc ii: “cc 15 
£6 12, (9 3.9 ce iad 125; ce 2 ce 14. 
73 11.8, cc 4 ce “ 113; ce ft: ce Dee 
“cc 10.8; cc 4 cc a3 ae ce 2 ce 25 
(zs Lt; iss 3.8 “ce ce is cc 2; i$ 13. 
ce th; (as 4 “cc ce 113; ce 2: ‘c DAF 
ce OZ: cc 4 ce cc 113; cc 2 as 30. 
Ho TOES i Eee a ese 1s gee cm ata 2 
e Cie ae ato rap f(a ee tore ls 
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: 
