Silurian Land Plants from Ohio. — Foerste. 135 
the Huffman specimen having surface markings shows only 
the septa, although a second much larger fragment of an in- 
terior east indicates both the septa and the siphuncle; the 
Eaton specimen shows neither, being only a cast of the ex- 
terior surface. 
The Brown specimens in the writer's possession show a 
marked dorsal flattening of the shell, the sides being gently 
and the ventral surface strongly convex (See Fig. 7c, PI. vii, 
Proc. Boston Soe. Nat. Hist., Vol. xxiv). The Huffman speci- 
men at first sight seems to be quite different, since it does not 
show a very marked dorsal flattening or a marked though 
rounded angulation at the junction of the dorsal with the 
lateral surfaces. Indeed, the present curvatures would seem 
to indicate that the longer diameter was lateral and the shorter 
dorso- ventral. This is believed to be due to pressure modify- 
ing the original form of this fragment of the shell. Judging 
by the shape of the scars, as further explained in the next 
paragraph, the real form of the shell is found to be such as to 
warrant direct comparisons with the Brown specimens. The 
siphuncle, as in the latter, is apparently nearly central, a little 
nearer the dorsal side. The dorsal convexity is greater than 
the lateral, there being no marked angulation along the dorso- 
lateral region; instead of that, this is the region of the 
greatest convexity of the shell, though the curvature here is 
regular and not angulated. The lateral faces round into the 
ventral ones, as in the Brown specimens. The Eaton speci- 
men gives no ideas on the general form of the shell. 
The "scars" of the Brown specimens have an average width 
of 4 mm. on the dorsal surface, about 50 mm. above (toward 
the aperture) the point where the lateral diameter is about 
54 mm. and the dorso-ventral diameter 68 mm. The average 
width is 3.5 mm. on the ventral surface, where the lateral diani- 
eter of the shell is estimated to be 45 mm. and the dorso- 
ventral diameter is 56 mm. On the dorsal surface the scars 
have a greater length than on the lateral surface, owing to the 
greater arching of the lower crescentic boundaries, and to the 
fact that the upper extremities of these boundaries approach 
each other as a rule more closely anteriorly at the boundary 
of the next upper scars. The cause of this change of form 
is much better seen in the Huffman specimen. The crescentic 
