BRACHIOPODA 167 



valves. In Magellania (fig. 72 B, I) the loop extends nearly 

 to the anterior margin of the shell and is then bent back 

 upon itself. In many Palaeozoic and a few Mesozoic 

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

 ribbons; in Spirifer (fig. 82 A) the apices of the spirals 

 are directed towards the lateral margins of the shell, in 

 Glassia they point inwards, in Atrypa (fig. 83 A) upwards 

 to the centre of the dorsal surface. The brachial skeleton 

 is absent in all the inarticulate genera, as well as in some 

 of the articulate forms such as Productus and Ghonetes. 



The development of the brachial skeleton has been 

 studied in some living species of Terebratulina, Magel- 

 lania, and Terebratella. In Terebratulina the adult form 

 is reached almost directly ; but in Magellania the brachial 

 skeleton passes through various stages before the adult 

 condition is attained ; and it is noteworthy that these 

 stages are similar to the adult forms of certain other 

 genera. Thus in Magellania venosa the brachial skeleton 

 passes through stages which, in succession, resemble the 

 brachial skeletons of the genera Gwynia, Cistella, Bou- 

 chardia, Megerlina, Magas, Magasella, and Terebratella, 

 after which the adult condition is reached. Another 

 striking fact is that some species, which have hitherto been 

 referred to the genus Magellania, have a development 

 differing from this; thus M. cranium passes through 

 stages distinctive of the genera Gwynia, Cistella, Platidia, 

 Ismenia, Milhlfeldtia, and Terebratella. If the stages 

 through which an individual passes in its development be 

 taken to indicate its ancestry, then it follows that in 

 Magellania there are two groups of species having different 

 ancestors, and these two groups must therefore be regarded 

 as constituting two distinct genera (see pages 13, 14). 



The largest brachiopod known is Productus giganteus, 



