10 INTRODUCTION. 



is next continued forwards from the basal fold to the transverse end of the loop, and is 

 thence continued forwards, independently of the loop, between the bases of the spiral por- 

 tions of the arms, uniting the right to the left throughout the extent of the spire, forming 

 the ' spiral fold,' y, pi. 2, fig. i, of the brachial aponeurosis. 



The basal fold, w, pi- 2, fig. i, is perforated by the oesophagus, the mouth opening into 

 the pallial cavity below the beginning of the spiral fold : the alimentary canal bends back- 

 wards, over the basal fold, after perforating it ; the fold thus intervening between the 

 oesophagus and intestine. The filaments given off from the beginning of the arms, 

 supported by the basal fold, are shorter than the rest. The aponeurosis, reflected from 

 the lateral to the spiral folds, forms the foi'e-part, v, v, pi. 3, figs, ii and iii, of the small 

 visceral cavity ; the pallial membrane is continued from the union of the lateral with the 

 basal folds, is stretched over the visceral interspace between the advancing crura, and is 

 then continued backwards towards the hinge, protecting the hearts and sinuses behind the 

 transverse fold. The lateral folds decrease in breadth as they advance forwards : the 

 spiral fold, y, pi. 2, fig. 1 , which, at its beginning, is broader than the broadest part of the 

 lateral folds, gradually becomes narrower as it approaches the termination of the spire. The 

 lamellae of the several above-defined parts of the aponeurotic supporting and connecting 

 brachial folds, separate when they reach the calcareous plate, a, and the hollow muscular 

 stems, z, z, pi. 3, fig. ii, of the fringed arms, closely surround and adhere to these stems, 

 being continued thence upon the border formed by the roots of the fringe-filaments, 

 where the aponeurotic character is exchanged for that of a delicate membrane, which is 

 finally lost upon the filaments themselves. 



The fibres of the muscular walls of the brachial canal, pi. 3, fig. ii, -, are beautifully 

 arranged in a decussating double spiral, evidently adapted for compressing the contained 

 fluid, and thereby reacting upon the arm of which the muscular canal forms the base. In 

 the TerehratulcB, like Ter. psittacea, with free multispiral brachia, the fluid of the canal 

 being acted upon by the spirally-disposed muscles composing its parietes, is forcibly injected 

 towards the extremity of the arm, which is thus unfolded and protruded outwards. In 

 the species resembling Ter. Jlavescens, the spiral portion may also be, in like manner, so 

 far unfolded as to react upon the closed valves of the shell. 



What power the animal may possess of further unfolding and protruding the free 

 extremity of the united spiral portions of the arms, can only be determined after careful 

 observation of the living Terebratulce in their native localities. The structure of the parts 

 in question led me to note, in 1833, the important difference between Lingula and those 

 species of Terehratula which resemble Ter. chilensis in the structure of the arms, " since, 

 from their attachments, they are fixed and cannot be unfolded outwards as in Lingula."^ 

 Mr. Davidson has expressed a similar opinion in his comprehensive and able " Classifica- 

 tion of recent Brachiopoda,"^ in reference to the spiral portion of the fringed arms ; 



^ Zool. Trans., vol. i, p. 149. 



2 Annals and Mag. of Nat. Hist., t. ix, 2d ser., 1852, p. 363. 



