Strophomena and Other Fossils 
137 
on the exterior surface, although the interior frequently is marked 
anteriorly by very fine striae which are slightly divergent and 
would be called radiate if they extended farther back. The lar- 
gest specimiens attain a length of 19 mm. In these specimens 
the thickness of the shell equals 4 mm., the valves being equally 
convex. In width, the specimens vary from 12.5 mm. to 15 
and even 16 mm. There is a resultant variation in outline, the 
former being ovate-obloiig, and the latter more broadly ovate 
in outline. The difference obviously is one of age, the older shells 
becoming rapidly broader anteriorly but not postero-laterally. 
For a length of about 7 or 8 mm. the lateral outlines are but 
slightly convex- Posterior to this part of the outline, the sides 
converge more rapidly to the rapidly-rounded outline at the beak, 
and anteriorly, the antero-lateral outline also is rather strongly 
rounded, compared with the moderate convexity of the anterior 
margin. The shell is rather evenly convex within the limits of 
the dimension stated. Even anteriorly the thickness of the shell 
is fairly well maintained to within 5 or 6 mm. from the anterior 
margin. 
Helicotoma brocki, sp. nov. 
{Plate X, Fig. 11; Plate XI, Fig. 3) 
Helicotoma brocki belongs to the group of species typified by 
Helicotoma planulata, in which a series of revolving striae is 
found on the lower half of the outer side of the whorls of the shell. 
These revolving striae, however, are much stronger than in 
Helicotoma planulata, and in consequence, the lower half of the 
whorl appears more protuberant than in the latter species. There 
are five revolving striae; of these, two occupy the most promi- 
nent part of the lower half of the whorl, with the striation above and 
that below of equal size and an equal distance apart. The fifth 
striation is less prominent; it forms the outer edge of the flattened 
base of the shell, and the distance between it and the striation 
immediately above is less than betw’-een the other striae. The 
under side of the whorls is strongly flattened or slightly concave, 
with a rise of the surface at the margin of the umbilicus. Trans- 
verse striae very fine, but distinctly seen under a lens. On the 
lower surface of the whorls they form an angle of about 50 degrees 
with the outer margin; on the outer sides they form an angle of 
about 30 degrees with the perpendicular; and on the upper side, 
