158 PATAGONIAN EXPEDITIONS I PAL/EONTOLOGY. 



1899 D. pat. v. Ihering, in: N. Jahrb. Miner., etc., v. 2, p. 24. 



1900 D. sulc. 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 (i 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 i 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. sulcosnm 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 i 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/), inajits 

 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 11-14 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. 



