BRACHIOPODA. 
347 
spire-bearing forms derived their brachidia from a primitive terebratuloid con¬ 
dition, and this derivation has been effected by growth with accompanying 
resorption. The progressive modification of the loop in the recent terebratellids 
by resorption of calcareous tissue in the growth of the individual, is a well- 
known fact which has invited the study of many investigators. In such forms 
this modification is extreme, and is unquestionably complicated by the intimate 
connection of the loop with the median septum of the brachial valve. With 
the single exception of Tropidoleptus, among the palaeozoic genera, there is no 
clear evidence that the median septum has shared in, or contributed to, the 
growth-modification of the brachial supports; nevertheless, the outcome and 
final result of this growth with modification in the most progressed forms of 
Terebratella and such palaeozoic genera as Dielasma, Cryptonella, Harttina, 
etc., is the sam.e. 
Progressive modification of the brachial supports in both the Helicopegmata 
and palaeozoic Ancylobrachia being now fully established, it is interesting to 
observe that the primitive condition of the loop, as in Dielasma turgida, is one 
of simple apposition of the two short brachial processes, at their expanded 
anterior extremities; having the expression of the mature loop in the genera 
Centronella, Rensselaeria, Selenella, etc. A simple step further back would 
afford a condition in which the brachial processes with their expanded extrem¬ 
ities are not as yet united, but discrete as in the rhynchonellids. A more 
primitive condition than that in Centronella or the centronelli’d stage in 
Dielasma, could not be different from this. On the ground of these differences 
in the conditions of the brachidium and the phyletic stages corresponding 
thereto, it would seem fair to infer that, of the rhynchonellids, the terebratu- 
loids and the spire-bearers, the first is the primitive stock, and the spire- 
bearers legitimate derivations from that stock, through the terebratuloids, or 
both of the latter derived along divergent lines from tbe rhynchonellids. This 
conclusion, however coherent and consistent with the geological evidence, will 
be found to lack stability until the data are sufficient to establish the fact that 
the brachia themselves, and not alone their calcareous supports, have passed 
through corresponding phases of growth and derivation. This latter question 
