MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 151 
posing the parietes of the canal, is forcibly injected towards the extremity of the arm, 
which is thus unfolded and protruded outwards. 
My opinion of the uses of the complex internal testaceous apparatus of Ter. Chilensis 
and its congeners, was at first shaken by observing that it was wanting in the more 
globose species, as Ter. vitrea, Ter. rubicunda, and Ter. psittacea, where it ought rather to 
have been proportionately developed, in order to act upon the valves. But the re- 
lations of the soft parts to the loop, as exhibited in Ter. Chilensis, showed that the 
increase of that part requisite to perform the same office in the globose species, would 
have been incompatible with the limited proportion of the soft parts which characterizes 
the genus Terebratula, and the opening of the shell is therefore effected by other means. 
It is interesting to observe that the globose figure is assumed by those species which 
have the weakest valves, in order to enable them to resist surrounding pressure, while 
in Ter. dentata, Ter. dorsata, Ter. Sowerbu, and Ter. Chilensis, in which the imperforate 
valve is more or less flattened, the whole shell is characterized by its superior thick- 
ness and strength. 
Under the microscope the brachial cilia are seen to be of a transparent horny texture; 
and the muscular stem to have no vascular trunk accompanying it, which would cer- 
tainly be the case if the blood of the animal was distributed through this part for its 
renovation: so that the absence of the adequate organization in the brachia, together 
with the above-described structure of the mantle-lobes, leaves no doubt as to the true 
position of the branchie. 
Although the pedicle is the fixed point to which the muscles are attached, I shall 
consider the extremities going to that part as the insertions. Two pairs of muscles 
arise from each valve. Those of the imperforate valve arise at a distance from each 
other: the anterior pair come off just behind the middle of the valve, fleshy ; they soon 
diminish to thin shining tendons, which converge and unite below the stomach, and 
then again separate and pass through the foramen of the perforated valve to be inserted 
in the pedicle. The posterior pair are very short, and wholly carneous ; they arise 
from the lateral depressions in the base of the central process of the hinge, and are 
inserted into the pedicle. The muscles of the perforated valve arise close together, 
so as to leave only a single muscular impression on either side'. The anterior pair 
soon diminish to slender tendons, which are inserted into the base of the imperforate 
valve ; the posterior pair pass exclusively to the pedicle. 
The pedicle is surrounded, except where it is attached to foreign substances, by a 
tubular prolongation of the superior lobe of the mantle. When this membrane is de- 
tached, the surface beneath is found to be smooth ; and the fibres, when separated, ex- 
hibit some of the lustre of the tendons of which it seems to be composed. At its ex- 
tremity these become partially decomposed, are of a black colour, and separated irre- 
gularly from each other, so as to form an expanded base of attachment. 
! Fig. 3. 
