I2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
A single specimen (U.S.N.M. No. 127205h) measuring 7.9 mm. 
in length and 8.3 mm. wide is unique in showing unequivocally the 
two descending branches free of each other (pl. 2, I, fig. 21). In this 
specimen the jugum has been resorbed because no evidence of break- 
age on the descending ribbons can be seen. This is the ultimate state 
in the development of the loop. 
Stages 8.1 to 9.0 mm.: A single specimen with loop (U.S.N.M. 
No. 127204e, not figured) representing these stages measures 8.6 mm. 
long by 8.4 mm. wide and is one of the largest specimens in the col- 
lection. The loop is 5.6 mm. long and the hood is 3.7 mm. long by 
1.9 mm. wide. The hood is only moderately notched on the posterior 
side. The descending lamellae lie very close together but the jugum 
has been resorbed and the descending branches are free, the ultimate 
condition for the loop. If larger specimens exist it is likely that the 
only further change in the loop would be gradual lateral migration of 
the descending lamellae so that they would be more distantly spaced 
and subparallel. 
Median septum (or ridge): No distinct septum or median ridge 
was seen in specimens from I.0 mm. to 3.5 mm. After the latter length 
is attained a median ridge is discernible but even in large adults it is 
not a conspicuous feature of the shell. It is apparent that the ridge or 
septum never shared in the development of the loop as it does in so 
many modern long-looped brachiopods. 
Summary of loop development of Cryptacanthia.—The development 
of the loop in stages below 1 mm. can only be inferred from the early 
stage in the loop development of other brachiopods. In the earliest 
stages it is postulated that the loop originated as two processes grow- 
ing anteriorly from the hinge region, the descending branches de- 
veloping an angular bend medially and finally uniting distally at about 
the I mm. stage. The descending lamellae in these stages would be 
thin and delicate and no echmidium would have been formed (pre- 
centronelliform stage). 
After the 1 mm. stage the loop begins to thicken and at the place 
of junction of the distal ends of the descending lamellae the joined 
elements widen and flatten to form a plate having the shape of a spear- 
head and here called the echmidium (centronelliform stage). After 
the formation of the central echmidium the loop continues to 
strengthen, but at the anterior tip of the echmidium a bud appears in 
the form of a small elliptical cup. This is the incipient hood (early 
cryptacanthiform stage). In succeeding stages this structure expands 
and elongates with the growing loop. After the appearance of the 
hood the echmidium cleaves medially, the split lengthening and widen- 
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