Review of Recent Geological Literature, 395 
It has also been tacitly assumed, that the variations in the form of 
the loop which are known to occur in recent terebratuloids during the 
process of individual growth, were not manifested by palaeozoic loop- 
bearing species. Both of these important points, which, up to the time 
of publication of this remarkable paper were lacuna* in our knowledge, 
have now been most admirably and conclusively covered. 
Considering the latter subject first, the authors have shown by means 
of a series of preparations of the well-known Terebratula, or Dielasma 
turgida Hall, from the St. Louis limestone, that the loop in the earliest 
observed growth stage (length of shell .8 mm.) consists of a simple coa- 
lescence of the crural processes forming, at their anterior extremities 
a hinge-plate of precisely the form of that of the mature Centronella 
and essentially like that of Rensselozria. With later growth a resorp- 
tion of the anterior portion of the triangular plate immediately sets in, 
and soon results in the normal form of the loop in Dielasma, which is 
shorter than in the majority of palaeozoic terebratuloids, though con- 
structed on the same plan. Thus the important facts are established: 
1st, that the loops of paheozoic terebratuloids are subject to metamor- 
phoses, and 2d, that Centronella and Rensselozria are primitive terebra- 
tuloid types, the earlier condition of whose brachial supports could not 
have differed from the discrete crural processes of the rhynchonelloids; 
and (though the authors make no such inference) that the Ancylobra- 
chia as a whole, have been derived from this much earlier stock. The 
Silurian shells which have been supposed to possess long, reflected 
loops, and have been referred to the genera Waldheimia and Halliiia, 
have been shown to be spiriferous. 
A series of preparations of the brachial supports in early conditions 
of Zygospira recurvirostra, a Trenton limestone species, have furnished 
even more surprising results. In the earliest observed phase, the cal- 
cined support is in the secondary centronelliform stage, shown in Die 
lasma; that is, consists of two simple crural extensions, coalesced and 
expanded anteriorly, and has the apical portion of the plate resorbed. 
In the process of growth the progressive modification of the plate by re- 
sorption produces, first, the Diclasma-stagc, and subsequently a con- 
tinued growth of each lateral process forward, beyond the angles made 
with the transverse band, results in a slight upward curvature of these 
lateral processes, then in a single complete revolution, which is the 
stage where this growth ceases in Hallina. Acontinuation of this spi- 
ral growth affects the completed spirals of Zygospira, with introverted 
apices and broad jugal band. The transverse band or ascending lamel- 
la? of the loop in Dielasma and the jugal band {jugum | of Zygospira are 
thus shown to be similar remnants of the primitive centronellid plate. 
The Helicopegmata (spire- bearing brachiopods;, or at least the division 
of them with introverted spirals [Atrypidm) are forms attaining the 
brachidium by a series of metamorphoses and deriving their origin 
from the Aneylobrachia. "A further natural conclusion is that the 
Ancylobrachia are older and more primitive than the Helicopegmata." 
and that both have had their inception in the Rhynchonellida . 
