MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 149 
Ter. dorsata, Ter. dentata, Lam., and Ter. Sowerbii, the reflected portion is consider- 
able. The loop, besides being fixed by its origins or crura, is commonly attached to 
two processes going off at right angles from the sides, or formed by a bifurcation of the 
extremity, of a central process, which is continued forwards to a greater or less extent 
from the hinge; but it is sometimes entirely free, except at its origins, as, e. g., in 
Ter. vitrea. This reflected loop, forming two arches on either side the mesial plane, 
towards which their concavities are directed, I have figured as it exists in Ter. Chi- 
lensis' and Ter. Sowerbii?. It is represented of a similarly perfect form in Ter. dentata, 
by M. de Blainville in his ‘ Malacologie’’: and the same apparatus in Ter. dorsata is 
very well figured by Chemnitz‘; by Sowerby®; and more recently by G. Fischer de 
Waldheim®. A similar form is also figured in another species of Terebratula by Poli’. 
The arches of the loop are so slender, that, notwithstanding their calcareous nature, 
they possess a slight degree of elasticity, and yield a little to pressure; but, for the 
same reason, they readily break if the experiment be not made with due caution. The 
interspace between the two folds of the calcareous loop is filled up by a strong but 
extensile membrane, which binds them together, and forms a protecting wall to the 
viscera: the space between the bifurcated process in Ter. Chilensis is also similarly oc- 
cupied by a strong aponeurosis. In this species the muscular stem of each arm is 
attached to the outer sides of the loop and the intervening membrane. They com- 
mence at the pointed processes at the origins of the loop, advance along the lower 
portion, turn round upon the upper one, and are continued along it till they reach the 
transverse connecting bar, where they advance again forwards, and terminate by making 
a half spiral twist in front of the mouth. It is these free extremities which form the 
third arm mentioned by Cuvier’. These arms are ciliate on their outer side for their 
entire length ; but the cilia are longer and much finer than the brachial fringes of Lin- 
gula ; and except at the extreme ends, which have a slight incurvation, they are uni- 
formly straight. There is thus an important difference between Lingula and those 
species of Terebratula which resemble Ter. Chilensis in the powers of motion with which 
the arms are endowed; since from their attachment to the calcareous loop they are 
fixed, and cannot be unfolded outwards as in Lingula. Owing to this mode of con- 
nexion, and their ciliated structure, their true nature was much more liable to be mis- 
taken by the early observers, though it appears not to have escaped the discrimination of 
Linnzus, who, as Cuvier has observed, founded his character? of the animal of Anomia 
on the organization of one of the Terebratule, which he included in that genus. 
| Fig. 4. 2 Fig. 16: 3 Pl. li. La. * Conchyl. Cabinet, band viii. tab. bexviii. fig. 711. 
5 Genera of Shells. 6 Notice.sur le Charpente Osseux des Térébratules, fig. 3. 
7 Testacea utriusque Siciliz, vol. ii. pl. 16. 8 Régne Anim., (nouy: ed.) tom. iii. p. 171. 
9 Anomra. Animal Corpus Ligula emarginata ciliata, ciliis valvule superiori affivis. Brachiis 2, linearibus, cor- 
pore longioribus, conniventibus, porrectis, valvule alternis, utrinque ciliatis, ciliis affixis valvulis utrisque.—Syst. 
Nat., (ed. xii.) vol. i, pars 2. p. 1150. 
x2 
