CLASS II BRACHIOPODA 297 
formed by the union of the crural plates in the Pentameracea. It serves 
for the attachment of muscles, and may either rest upon the bottom of the 
valve, or may be supported by a median septum. When the crura remain 
separate, and are therefore not for muscular insertion, they are homologous 
with and the equivalent of the crura in the khynchonellidae. 
In the Spiriferacea, two thin, spirally coiled ribbons, or spiralia, are 
attached to the crura; the coils exhibiting great diversity in form, in the 
number of volutions, and in the direction of the hollow cones (Fig. 494, 
B,C, D). The spiralia are usually joined by a transverse band or jugui (Fig. 
494, D). When the latter is discontinuous, the parts are called the jugal 
processes. The bifurcations of the jugum may enter between the convolu- 
tions of the spiralia, and may be continuous with them to their outer ends, 
forming what is termed a double spiral or diplospire (Fig. 494, B). In the 
Terebratulidae, the brachia are also attached to the crura, and form free, 
shorter, or longer loops, which depend toward the anterior margin (Fig. 494, 
E, F, G). The two descending branches may either unite directly or may be 
joined by a transverse band ; or the descending branches may recurve, continue 
upward as ascending branches, and be connected posteriorly by a transverse 
band. In the Terebratellidae, during all or some portion of the animal’s 
existence, the loops are attached to a median septum by outgrowths from 
the descending lamellae. In the Stringocephalidae and Megathyrinae (Fig. 
494, H), the descending branches are parallel to the lateral margins of the 
shell, and unite along the median line; but in some degenerate species, the 
loop is represented by a transverse band situated centrally on the median 
septum. The entire form of the brachidia is manifestly dependent upon the 
character of the convolutions of the fleshy arms. In recent Lhynchonellae 
(Fig. 494, A) the brachia form hollow spiral cones, and if we imagine these 
as supported by a calcareous framework, the result will be the form of 
support seen in the Aftrypidae. The fleshy arms of the Terebratellidae are 
continuous with, and have at first the form of the loop, but later develop a 
coiled median arm. Here the loops only have calcareous supports ; but in 
the Spiriferacea, the entire brachia are provided with an internal calcareous 
skeleton. 
The changes in the form of the brachidia in the Telotremata during the 
ontogenetic stages of the individual furnish very important data in regard 
to the relationships existing between the different groups. In the Sporv- 
feracea, not only do the number of convolutions of the spirals increase with 
age, but the brachidia begin with Centronella- and Dielasma-like loops, from 
the outer ends of which the spires are developed. Still more striking are 
the metamorphoses which the loops of the Terebratulacea undergo. According 
to Oehlert and Beecher, the loop of the living austral genus Magellania passes 
through stages which correspond successively to those of Gwynia, Cistella, 
Bouchardia, Magas, Magasella, Terebratella, and Magellania ; and Friele has 
shown that the metamorphoses of the loop in the boreal form Macandrevia 
cranium correspond in succession to the genera Platidia, Ismenia, Mihlfeldtia, 
Terebratalia, and Macandrevia. 
A knowledge of the character of the brachidia in the Spiriferacea and 
Terebratulacea is almost always requisite for critical generic determinations. 
But an examination of the interior of the shell in fossil Brachiopods often 
involves great difficulties, owing to the infiltration of calcite, or the filling up 
