160 MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 
termediate to the Lamellibranchiate and Tunicate orders; not, however, possessing, so 
far as they are at present known, distinctive characters of sufficient importance to 
justify their being regarded as a distinct class of Mollusks, but forming a separate 
group of equal value with the Lamellibranchiata. 
PLATE XXII. 
Anatomy of Terebratula. 
Fig. 1. Terebratula Chilensis, Brod., natural size. 
2. Terebratula Uva, Brod., natural size. 
3. Perforate valve of Terebratula Chilensis. 
a. The perforation, or groove, through which the tendons of the muscles 
pass to form the pedicle. 
b. The teeth of the hinge, which are locked in the depressions 0. b. of fig. 4. 
c. The muscular impressions. 
4. Imperforate valve of the same specimen. 
a. The middle depression of the hinge. 
b. b. The lateral depressions. 
c. The mesial process, or ridge continued from the hinge. 
d. The lateral processes of the same, which are attached to the bends of 
e.e. The elastic calcareous loop. 
f.f. Small processes at the origins of the crura of the loop. 
g.g- Impressions of the anterior pair of muscles. 
h. h. Impressions of the posterior pair. 
Figg. 5. to 9. are dissections of a smaller specimen of Ter. Chilensis magnified two 
diameters. 
5. The soft parts corresponding to the perforate valve. 
6. The soft parts corresponding to the imperforate valve. The branchial ves- 
sels, brachial filaments, and liver, may be distinguished through the semi- 
transparent mantle. 
7. The soft parts viewed from the same aspect as in Fig. 5, with the mantle- 
lobe reflected, showing more distinctly the branchial vessels, and ex- 
posing the arms in their natural position. 
8. The soft parts viewed from the same aspect as in Fig. 6., with the mantle- 
lobe reflected, exposing part of the visceral mass, and the bend of the 
arms following the bend of the calcareous loop. 
9. The mantle-lobes further separated, the calcareous loop broken through and 
removed, and the arms dissected off and displaced, to show the decus- 
sation of the muscles, and the small visceral mass. 
