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 

 v.ilve, or may be supported by a median septum. When the crura remain 

 .-rparate, and are therefore not for muscular insertion, they are homologous 

 with and the equivalent of the crura in the Rhynchonettidae. 



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 jugum (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 Rhynchvnellae 

 (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 Atrypidae. 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 Spiri- 

 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, 

 Boucliardia, Magas, Magasella, Terebratella, and Magellania; and Friele has 

 shown that the metamorphoses of the loop in the boreal form Macandrema 

 criniiiim correspond in succession to the genera Platidia, Ismenia, Milhlfeldtia, 

 Terebratalia, and Macandrema. 



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 



