i 45. BRACHIOPODA OF THE CARBONIFEROUS SYSTEM. 737 



where the shell is removed, these organs may be seen in 

 their original situation. 



The following description, by Professor Owen, of a 

 recent animal of the same family, a native of the South 

 Seas, explains the nature of this structure. 



" The loop- like processes observable in the interior of the shells of 

 many of the fossil terebratula?, are the internal skeleton, and are for 

 the attachment of the muscular stems of the arms. In Terebratida. 

 psittacea* a recent species (Lign 16$, fig. 6), two spiral arms, fringed 

 at their outer margins, are seen to arise from these processes ; these 

 arms are quite free, except at their origins : when unfolded, they are 

 twice as long as the shell, and in a state of contraction are disposed 

 in six or seven spiral gyrations, which decrease towards their extre- 

 mities. The mechanism by which the arms are extended is most 

 beautiful and simple : the stems are hollow from one end to the other, 

 and filled with fluid, which being acted upon by the spirally disposed 

 muscles composing the walls of the canal, is forcibly injected towards 

 the extremity of the arms, which are thus unfolded and protruded. 

 The spiral disposition of the arms is common to the whole of the 

 brachiopodous genera, whose organization has hitherto been examined ; 

 and it is therefore probable, that in the fossil genus Spirifer, the 

 entire brachia were similarly disposed, and that the internal, calca- 

 reous, spiral appendages were their supports. If indeed the brachia 

 of Terebratula psittacea had been so sustained, the species would have 

 presented in a fossil state an internal structure very similar to that 

 of Spirifer. "f 



Cejyhalopoda. — The cephalopodous shells found in the 

 mountain limestone and associated strata, amount to up- 

 wards of sixty species ; the Ammonites of this system are 

 of a peculiar kind (Goniatites, Medals, p. 494). There 

 are several species of Better ojohon {Lign. 168, Jig. 4), 



* In the Mediterranean, at the depth of 100 fathoms, there is a pro- 

 fusion of one living species of Terabratula: and also a species of 

 Orthis, which is one of the most ancient generic types of brachiopoda, 

 several species occurring low down in the Silurian deposits. In the col- 

 lection of shells in the British Museum, there is a slab of stone with 

 some twenty or thirty recent Terebratulae attached, from New Holland. 



f Frofessor Owen, on the Anatomy of the Brachiopoda ; Zoological 

 Transactions, vol. i. p 145. 



