Io SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
midium is 0.7 mm. long and the hood varies from 0.35 to 0.5 mm. in 
width. The conical hood tapers rapidly in a posterodorsal direction. 
Stages 4.1 to 5.0 mm.: Twelve specimens (U.S.N.M. Nos. 
127203r-z, 127204c, d, h) in this size range exhibit the loop in vari- 
ous stages of completeness. The loop varies in length from 2.2 to 
2.9 mm. but averages about 2.4 mm. The echmidium varies from 0.8 
to 1.8 mm. in length and has a maximum width of 0.64 mm. Specimen 
U.S.N.M. No. 127204h is 4.8 mm. long and has one of the best pre- 
served loops in this interval (pl. 2, F, figs. 10, 11). The descending 
lamellae are broad and flatten anteriorly on the echmidium and be- 
come nearly vertical. At the anterior end they flare laterally and unite 
with the lateral walls of the hood. The ventral margin of the hood 
slopes rapidly posterodorsally, the hood narrowing in that direction. 
The posterior surface of the hood is short and narrowly convex. Thus 
in ventral view it expands laterally but with a deep reentrant dorsally. 
The lateral flaring of the outer walls of the hood is the beginning of 
the anterior cleft of the echmidium so prominent in succeeding stages. 
In this specimen the cleft is 0.5 mm. long. The loop at this 4.1 to 
5.0 mm. stage is thus an adult one but the anterior cleft is short. The 
cleft in the anterior point of the echmidium in a smaller specimen 
(U.S.N.M. No. 127203r) in this range is small and is noticeable only 
in the hood. The cleavage of the echmidium is thus initiated when 
the shell has attained slightly more than 4 mm. of length. Specimens 
of 5 mm. length show a deep cleft in the hood, about I mm. deep in 
specimen U.S.N.M. No. 127203z. The hood in this specimen is 
1.5 mm. long and I mm. wide. 
Stages 5.1 to 6.0 mm.: Four specimens (U.S.N.M. Nos. 127204k, 
m, t, u) in this group preserve the loop in most of its details. It is 
substantially the same as that of the late stages of the previous group 
but some modifications are evident. The cleft in the echmidium is 
now deeper, almost half the length of the loop which varies from 3.1 
mm. to 3.6 mm. in length. The junction of the lateral branches is 
short and the reentrant in the hood on the posterior side is deeper in 
most instances but this appears to be a variable feature. The spines 
on the anterodorsal side of the loop are long and in two rows anterior 
to the junction of the lateral branches. The longest spine measures 
0.4 mm. in length. The hinge plate in all members of this group is 
complete and usually perforate. In specimen U.S.N.M. No. 127204t, 
the loop is 3.6 mm. long, the hood is 1.9 mm. long, the cleft is 1.3 mm. 
deep, and the hood is 1.2 mm. wide (pl. 1, I, figs. 26, 27; pl. 2, G, 
figs. 13, 14). 
Stages 6.1 to 7.0 mm.: Five specimens (U.S.N.M. Nos. 127204-0, 
