NO. 3 LOOP DEVELOPMENT OF CRYPTACANTHIA—COOPER 9 
the length of the echmidium averages 0.55 mm. The descending 
lamellae are thicker and broader than in the previous stages and the 
bend medially is less pronounced, but the form of the loop is essen- 
tially the same. The echmidium is broader and longer and now has a 
long, sharp point. The crural processes are somewhat more pro- 
nounced and have sharper points than in the preceding stages. Some 
variation exists in the stoutness of all the elements, but generally they 
are stronger than the preceding. No ascending elements appear in 
these stages but traces of them are suspected. In two instances 
(U.S.N.M. Nos. 127202s, w) a ridge along the center of the ech- 
midium is suggestive of an incipient development of the hood. 
Stages 3.1 to 3.5 mm.: Seven specimens in this range preserve the 
loop (U.S.N.M. Nos. 127202], 127203g-k, 127206k). In these the 
loop averages 1.48 mm. in length and the echmidium averages 0.65 
mm. long. The shortest loop is 1.4 mm. long in a specimen 3.1 mm. 
long and is 1.6 mm. long in a specimen 3.5 mm. long. This interval 
sees the appearance of the hood. The best specimen (U.S.N.M. No. 
1272031) to show this is about 3.2 mm. long with a loop 1.5 mm. long 
and with an echmidium 0.8 mm. long which is remarkable for the 
attenuation of the anterior point and the presence on this point of 
the first bud of the hood (pl. 2, D, fig. 7). This is 0.56 mm. long 
measured in the direction of shell length and is 0.24 mm. in height 
measured at right angles to the length. The line of contact with the 
echmidium is 0.24 mm. long and is located at the very end of the at- 
tenuated tip. The delicacy of this structure and its flimsy contact with 
the echmidium evidently account for the rarity of its preservation. 
The ventral face of the hood cannot be directly seen but the hood 
from the dorsal side appears to be elliptical and must therefore form 
an elliptical opening on the ventral side. From the dorsal side the 
echmidium shows well the suture between the two descending ele- 
ments at the line of junction. At the anterior tip of the echmidium a 
faint trace of divergence laterally of the joined descending and as- 
cending elements foreshadows the deep cleft in later stages. 
Stages 3.6 to 4.0 mm.: Five specimens (U.S.N.M. Nos. 127203l, 
m,n, 0, p) exhibit these stages of the loop, which has become stout, 
and the echmidium long, wide, and pointed but still undivided. The 
loop varies in length from 1.6 to 1.8 mm. and the echmidium is 0.88 
to 1.0 mm. long. In two specimens (U.S.N.M. Nos. 127203n, 
127203-0) the latter plate is 0.60 mm. wide but is still undivided at 
its anterior tip. Two specimens (U.S.N.M. Nos. 1272031, m) show 
the hood, the former showing the posterior half (pl. 2, E, fig. 9) and 
the latter the anterior half. The attachment of the hood to the ech- 
