4<D PATAGONIAN EXPEDITIONS I PALEONTOLOGY. 



except on young specimens of 8 to 10 mm. or less, when they continue 

 across the venter; ribs in part simple, part branching once just before 

 reaching the middle of the flank and part consisting of short ribs on the 

 outer half of the whorl interpolated between the long ones. 



Sutures having the same general characteristics as in the preceding 

 species, with broad, not very deeply dissected, lobes and saddles. The 

 forms and proportions of the several lobes and saddles are nearly the 

 same in the two species, the principal difference being in the more com- 

 plicated and deeper dissection of the suture in H. patagonense, but this 

 difference is not as great as a comparison of the figures would indicate, 

 because both the sutures of H. argentinense are taken from much smaller 

 and less mature specimens than that of the other species, and the larger 

 one is considerably weathered. There is only one auxiliary lobe visible 

 on the flank, and the dorsal portion of the suture, as seen on a specimen 

 about 20 mm. in diameter, shows a slender, pointed and dentate, antisi- 

 phonal lobe and two rather simple, paired dorsal saddles on either side. 



Some variations in sculpture at different stages of growth have already 

 been indicated and, as the sculpture shows a decided tendency to become 

 less prominent on the larger specimens which are septate throughout, it is 

 possible that the last whorls of fully adult specimens become smooth as 

 in H. patagonense. 



Young shells also differ greatly in form from the adult, the umbilicus 

 being relatively larger (that is the shell less involute) and the whorls 

 much more convex, so that in shells 10 mm. or less in diameter the breadth 

 of the aperture is nearly or quite equal to the height, but these proportions 

 change rapidly to those of the adult. The following measurements of 

 four specimens indicate the normal proportions of the species and the 

 change from youth to maturity. The measurements under I are from the 

 largest specimen in the collection, which was too badly weathered for figur- 

 ing; those under II and III from the specimens represented by figures 4 

 and 5 on plate IX ; and those under IV from the inner whorls of a speci- 

 men that was broken down for the study of the early stages. 



I II III IV 



Diameter 128 122 48 10 



Umbilicus 28 25 11.5 3 



Height of aperture 68 65 23 4.8 



Breadth of aperture 37 35 14 4.2 



