NO. 3 LOOP DEVELOPMENT OF CRYPTACANTHIA—COOPER II 
q, u-w) with loops represent this stage. Anteriorly the lateral branches 
of the loop are distinctly separated. The hood is 1.6 mm. wide and 
the posterior surface is long, broadly rounded, and only moderately 
reentrant. The anterodorsal edge is marked by a double row of fairly 
strong spines. The loop varies in length from 3.6 mm. to 4.4 mm. and 
the maximum length of the hood is 2.8 mm. (pl. 2, H, figs. 16-18). 
Stages 7.1 to 8.0 mm.: One specimen only (U.S.N.M. No. 
1272041) in this range shows the loop (pl. 2, I, fig. 19). The speci- 
men is 7.5 mm. long and 7.2 mm. wide and its loop measures 4.4 mm. 
in length and the hood is 2.8 mm. long. The anterior spines are long. 
It is appropriate that at this place the adult loop be described because 
only one other change in it takes place. 
The loop of these stages has the crural bases hidden by the com- 
plete hinge plate which is buttressed against them. The descending 
branches are short and stout, only 0.7 mm. of broad, flat ribbon inter- 
vening between the hinge plate and the echmidium. The crural proc- 
esses are given off just anterior to the hinge plate and are short, 
bluntly pointed projections of 0.1 to 0.2mm. The descending lamellae 
are about 0.5 mm. wide. The echmidium is broad and deeply cleft 
anteriorly, 1.9 mm. in a loop 4.6 mm. long. The junction or jugum 
between the descending lamellae is 0.62 mm. long in the above loop, 
or less in others, and is a narrow plate. The descending lamellae pos- 
teriorly face each other and their narrow edge is perpendicular to the 
inner surfaces of the valves. Anteriorly these ribbons turn about go° 
to become parallel with the inner valve surfaces and at the same time 
become fairly deeply concave and narrow distally. The hood rests on 
the inner ventrally curved edge of the descending lamellae. Anteriorly 
the hood is a widely flaring cone, expanded above but narrowing dor- 
sally. It also narrows posterodorsally with an opening half or less of 
the anterior dimensions. The base of the hood is continued posteriorly 
beyond its narrow termination as two converging septa which unite 
to form a low ridge at the proximal end of the echmidium (pl. 2, I, 
fig. 20). The posterior side of the hood varies from round to nearly 
flat and with its anterior deeply notched (pl. 1, I, fig. 26) or with 
only a slight trace of indentation (pl. 2, I, fig. 20). The notch is a 
variable feature which appears to be more prominent and more uni- 
form in smaller stages. Seen from the side the descending lamella 
forms a keel along the dorsal side of the loop, the base of the hood 
is deeply concave and the hood itself bulges prominently in a lateral 
direction. The ventral surfaces of the descending lamellae are convex 
and spinose, one set of spines on the outer edge and the other coming 
off the inner convex face. 
