364 BRACHIOPODA. 



In Terebratula Chilensis, on the contrary, the movements of 

 the arms are extremely limited, and they can no longer be protrud- 

 ed from the shell as in the preceding species ; being connected 

 throughout their whole length with a peculiar complex testaceous 

 apparatus attached to the internal surface of the imperforate valve 

 of the shell (Jig- 171, B), the arrangement and uses of which are 

 thus described in the memoir above-mentioned. The principal 

 part of the internal framework alluded to consists of a slender flat- 

 tened, calcareous loop (y, f), the extremities of which are attached 

 to the lateral elevated ridges of the hinge : the crura of the loop di- 

 verge, but again approximate each other as they advance for a greater 

 or less distance towards the opposite margin of the valve; the loop 

 then suddenly turns towards the imperforate valve, and is bent back 



Fig. 171. 



upon itself for a greater or less extent in different species. The 

 loop, besides being fixed by its origins, or crura, is commonly 

 attached to two processes (</, d) going off at right angles from 

 the sides, or formed by a bifurcation of the extremity of a central 

 process (c), which is continued forwards from the hinge, but it is 

 sometimes entirely free except at its origins. 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 pres- 

 sure. 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 be- 

 tween the bifurcated processes in T. Chilensis is also similarly occu- 

 pied by a strong aponeurosis. In this species the muscular stem 

 of each arm is attached to the outer sides of the loop and the inter- 

 vening membrane. They commence at the pointed processes at the 

 origin of the loop, advance along the lower portion, turn round 

 upon the upper one, are continued along it till they reach the 



