of Recent Brachiopoda. 363 



careous process or loop for the support of the arms. These 

 vary in their dispositions and details in the different genera, but 

 could not be protruded from the shell as they are united together 

 by a membrane, which effectually maintains them in one invari- 

 able position ; they have no connection with the opening and 

 closing of the shell, as supposed by some authors, which is ac- 

 complished by special muscles passing from the centre of the 

 perforated valve to the fulcrum of the smaller valve ; thus in those 

 genera possessed of a calcareous loop the disposition of the 

 ciliated arms is not regulated or restrained by the form of the 

 loop, a point I was not convinced of before having entered into 

 the zoological details of these animals. 



I. Genus Terebratula, Lhwyd, 1698 (restricted). 



Shell usually oval, and convex, with the margins even or only 

 slightly waved ; valves articulating by means of two teeth in the 

 larger and corresponding sockets in the smaller valve : beak of 

 dental valve truncated and perforated by a circular foramen 

 partly completed by a deltidium formed of [one or two pieces. 

 Loop or calcareous appendage variable in length, formed of two 

 riband-shaped lamellae fixed to the crural base, and more or less 

 folded back on itself. The arms are partly supported by this 

 appendage, but they do not strictly follow the same course ; thus 

 the form of the arms is the same in Terebratula vitrea, with a 

 small loop, and in Terebratula australis which has a long one ; 

 the arms in both cases are united throughout by a membrane 

 forming three lobes, of which the central one is spirally folded 

 like the proboscis of a butterfly, but from its texture and rela- 

 tions it never could be moved" or unrolled at the will of the 

 animal ; the sole use of the folding of the arms is to give increased 

 surface for the disposition of the cilia. 



The letters in the diagrams of Terebratula and Terebratella are intended 

 to facilitate a comparison of the same parts. 



24* 



