NO. 3 LOOP DEVELOPMENT OF CRYPTACANTHIA—COOPER I5 
narrowing of the ascending elements will produce the loop of Glosso- 
thyropsis. 
The development of the external form of the two genera is also 
similar. Glossothyropsis is cryptacanthiform in its profiles and the 
disproportionate size of the two valves. The brachial valve of both 
genera is shallow. Glossothyropsis usually has a fairly strong median 
septum whereas that of Cryptacanthia is less well developed but far 
better developed than indicated by Girty. It is thus probable that 
Glossothyropsis is the ultimate stage of development of the cryp- 
tacanthiform loop. It is probable that the geologically youngest species 
of Glossothyropsis will be found with a loop advanced to the cryptonel- 
liform condition by narrowing of the ascending elements. 
COMPARISON OF THE LOOP STAGE OF CRYPTACANTHIA WITH 
LOOP STAGES OF OTHER TEREBRATULIDS 
The only known long-looped late Paleozoic brachiopods are Cryp- 
tacanthia, Glossothyropsis, Heterelasma, and an unnamed genus with 
cryptonelliform loop. The loop development of only the first one is 
now known. As described here Cryptacanthia passes first through a 
centronelliform stage (stages 1.5 to 3.5 mm.). The centronelliform 
stage characterizes a number of the earlier Paleozoic genera, such as 
Centronella, Nanothyris, Beachia, Oriskania, Rensselaeria and a few 
others (Cloud, 1942). No other adult loop is now known which 
represents the stages between 3.0 mm. and the point at which the 
jugum is completely absorbed. These stages of development of the 
hood are here called the cryptacanthiform loop. When the descending 
lamellae are free and the hood is represented by a broad ascending 
element with a broad transverse ribbon the loop is similar to that of 
Glossothyropsis and is called the glossothyropsiform stage. 
Comparison of these loop stages with the dallinid and terebratellid 
loops shows fundamental differences (Elliott, 1953). Both of the 
terebratellid families differ from Cryptacanthia in loop development in 
possessing a median septal pillar or septum in the initial stages. Cryp- 
tacanthia has only a modest median septum, and as revealed by 
C. prolifica this does not develop until the late stages of the ontogeny. 
Furthermore, the septum of Cryptacanthia is independent of the loop. 
In the terebratellids the median pillar is important because it is the 
site of development of the hood and median ring which produce the 
ascending elements of the adult loop. 
The loop-development stages of the Paleozoic Cryptacanthia thus 
bear little resemblance to the stages of development of the terebratel- 
lid genera even though the glossothyropsiform loop is similar to the 
