112 BRACHIOPODA. 



divided by a median line ; these muscles bifurcate before 

 reaching the dorsal valve and there form four impressions 

 (fig. 32 B, a, a'). In the Inarticulata the muscles are 

 much more complicated. 



The arms, already mentioned as occupying the main 

 part of the pallial chamber, are generally coiled up. In 

 some forms they can be protruded a greater or shorter 

 distance. Sometimes they are supported on a calcareous 

 framework the brachial skeleton which is attached to 

 the posterior part of the dorsal valve. In Rhynchonella it 

 consists of two short curved processes. In Terebratula 

 (fig. 42) the processes are longer, ribbon-like, and united 

 so as to form a short loop. In String ocephalus (fig. 43) 

 the loop is more extensive and forms a band parallel to 

 and near the margin of the valves. In Waldheimia (fig. 

 32 B) the loop extends nearly to the anterior margin of 

 the shell and is then bent back upon itself. In other 

 genera, the brachial skeleton is in the form of two spiral 

 ribbons; in Spirifera (fig. 38 A) the apices of the spirals 

 are directed towards the lateral margins of the shell, in 

 Glassia they point inwards, in Atrypa (fig. 39 A) upwards 

 to the centre of the dorsal surface. 



The largest brachiopod is Productus giganteus from 

 the Carboniferous Limestone, which has a breadth of 

 twelve inches ; the size varies from this down to about a 

 quarter of an inch. Generally the shell is very thin, but 

 in some forms (e.g. Productus llangollensis) it is thick 

 and massive, leaving but little space for the soft parts of 

 the animal. The external form varies immensely, it may 

 be globular, ovoid, hemispherical, quadrilateral, or tri- 

 angular. Usually both valves are convex, but in some, 

 one is plane the other convex, or one may be concave and 

 the other convex, and in the last case the space in the 



