158 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
1899 D. pat. v. Ihering, in: N. Jahrb. Miner., etc., v. 2, p. 24. 
1900 D. stile. Ortmann, in: Amer. Journ. Sci., v. 10, p. 380. 
Shell rounded, in young stage angular and curved, almost straight 
when old, with longitudinal ribs. Principal ribs, in young individuals, 
10 to 14, between which, in older ones, smaller ribs (1 to 3 in each 
interval) are intercalated, bringing up the total number to from 20 to 30 ; 
but always about 14 ribs are stronger than the rest. The ribs, which are 
rather sharp in the young stage, are rounded in older individuals; in 
young individuals the intervals are broader and flat, in older ones the 
ribs appear more crowded. In very old specimens the ribs are less dis- 
tinct, and are sometimes crossed by distinct lines of growth. 
Remarks: We possess numerous fragments, but no complete indi- 
vidual. This species grows very large: the largest diameter is 15 mm, 
while the smallest is 1 mm (upper end of the smallest fragment is 2.5 
mm in diameter, by a length of 18 mm). Between these small ones and 
the largest we possess all transitions. 
One of the smallest has only 9 ribs ; another one, as small as this one, 
has 10. Individuals of from 3 to 5 mm in diameter have 10 to 14 ribs: 
in all these very young ones the ribs are angular, the intervals flat, and 
the shell has a polygonal cross section. A fragment, diameter 7 mm, has 
15 principal ribs, and 3 very small ones intercalcated. At this diameter, 
from 6 to 10 mm, the characters of D. sulcosum Sow. are typically exhib- 
ited : about 14 ribs, with flat intervals, but sometimes a few intermediate 
ribs are present. An individual of from 6 to 9.5 mm diameter (on the 
lower and upper end respectively, length 54 mm), has on the narrow end 
1 1 large, and 1 small rib, on the wider end 12 larger, and 1 1 smaller (and be- 
sides a few striae), and represents thus, on the wider end, distinctly/), majus 
of Sowerby. In still larger fragments, more intermediate ribs are interca- 
lated, and at a diameter of from 1 1 to 12 mm there are about 14 larger, and 
14 to 15 smaller ribs. This intercalation of ribs is not regular : in some of 
the intervals there are none; in others 2 to 3. In very large fragments, 
the ribs often become indistinct, and further, the peculiar exfoliation of 
the outer layer of the shell tends in many individuals to obscure the 
ribs, especially the smaller ones, so that only 1 1 — 14 principal ribs are 
visible. Often distinct lines of growth are visible in larger fragments. 
The curvature of the young shell is a little stronger than that of the 
rest, which in most cases is almost straight, in some cases perfectly so. 
