14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
anteroventral direction. At their distal end they bear an unusually 
large hood with very short attachment to the descending lamellae. 
The hood is greatly enlarged posteriorly where it measures 1.7 mm. 
in length but narrows to its base of attachment where it is only 0.5 
mm. long. The hood is not attached posteriorly to the descending ele- 
ments. In ventral view the hood flares widely, 1.4 mm. wide, and the 
posterior edge is deeply notched. 
Comparison of the loop of C. compacta with that of C. prolifica— 
The loops of these two species are strikingly different. Posteriorly, 
that of C. compacta has more widely spaced descending lamellae and 
the jugum attaching these lamellae is longer and wider. The striking 
difference however is in the hoods. That of C. prolifica in the adult 
form has a long attachment and the posterior part is often extended 
as converging septa posteriorly along the inner edges of the descend- 
ing lamellae. The hood of C. compacta on the other hand is greatly 
expanded posteriorly and its attachment to the distal ends of the de- 
scending lamellae is very short. Only one loop of C. compacta was 
studied, but when this is compared with the many specimens of 
C. prolifica it seems evident that the loop of the Iowa species was pro- 
vided with more and longer spines than that of the New Mexico 
species. 
COMPARISON OF THE LOOP OF CRYPTACANTHIA WITH 
LOOPS OF OTHER PALEOZOIC GENERA 
The best-known long-looped Paleozoic brachiopod is Cryptonella 
whose loop typifies one major type of Paleozoic loop. It is like the 
end stage of both types of modern terebratellid loops. In Cryptonella 
the loop is simple in form and very slender in both ascending and 
descending elements. The development of this loop is not yet known 
but it is one of the most ancient of terebratulid loops as it occurs in 
the early Devonian. It is an odd fact that one of the most highly spe- 
cialized loops is actually one of the earliest. The loop of Cryptacanthia 
is not cryptonelliform although it simulates that loop in its late stages. 
The loop most like that of Cryptacanthia is that of Glossothyropsis. 
Although the loop of the type species of Glossothyropsis is yet un- 
known, other species have been taken from the Monos formation of 
Mexico and the Word formation of Texas which show well-preserved 
loops. The loop approaches the cryptonelliform loop in form, but the 
ascending elements are usually fairly broad and suggest the origin 
of the Glossothyropsis loop from that of Cryptacanthia. Elimination 
of the jugum joining the descending elements of Cryptacanthia and 
