202 MOLLUSCA. 



calcareous spires in the interior of the dorsal valve. In some 

 cases, the whorls of these spires are in turn furnished with 

 minute calcareous spines, showing that the cirri of the arms 



Fig. 142. Spirifera hysterica, Carboniferous. The right-hand figure shows the interior 

 of the dorsal valve, with the calcareous spires for the support of the arms. 



were also supported by an internal skeleton. The form and 

 development of the calcareous supports of the arms, though 

 liable to vary with age, nevertheless furnish important characters 

 in the discrimination of fossil Brachiopods. 



The Brachiopoda may be divided into two groups, called 

 respectively the Articulata and Inarticulata. In the former 

 the valves of the shell are united along a hinge-line, the lobes 

 of the mantle are not completely free, and the intestine ends 

 caecally. In this group are the recent Terebratulida and Rhyn- 

 chonellidcz. In the Inarticulata the valves of the shell are not 

 united along a hinge-line, the mantle-lobes are completely free, 

 and the intestine terminates in a distinct anus. In this group 

 are the Craniada, Discinida, and Lingulida. 



The Brachiopods are of such importance as fossils that the 

 characters of the more important groups, with their geological 

 range, and leading genera will here be given, along with figures 

 of the commoner types. As regards the general distribution 

 of the class in time, the Brachiopoda are found from the Cam- 

 brian Rocks up to the present day, and present us with an 

 example of a group which appears to be slowly dying out. 

 Nearly two thousand extinct species have been described, and 

 the class appears to have attained its maximum in the Silurian 

 epoch, which is, for this reason, sometimes called the " Age 

 of Brachiopods." Numerous genera and species are found also 

 in both the Devonian and Carboniferous formations. In the 

 Secondary Rocks Brachiopoda are still abundant, though less 

 so than in the Palaeozoic period. In the Tertiary epoch a still 

 further diminution takes place, and at the present day we are 

 not acquainted with a hundred living forms. Of the families 

 of Brachiopoda, the Productidcs, Strophomenidce^ and Spiriferidce. 

 are the more important extinct types. Of the genera, the most 



