394 MANUAL OF ZOOLOGY. 



In the first of these orders (Inarticulatd), the valves of the shell are not 

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

 intestine terminates in a distinct anus. In this division are included the 

 three families of the Craniate, Disdnida:, and Lingnlida all very ancient, 

 and all represented at the present day by living forms together with the 

 Silurian family of the Trimerellida. 



In the second order (Articulata), the valves of the shell are united by 

 teeth along the hinge-line, the lobes of the mantle are not completely free, 

 and the intestine ends blindly. In this division are included the living 

 families of the Terebratulida, Rhynchonellida, and the Thecidiida, and the 

 extinct families of the Spiriferidtz, Pentameridtz, Strophomenidce, and Pro- 

 ductidce. In the first two of these families the arms are supported upon a 

 shelly loop, of variable shape and size (fig. 204, B) ; whilst in some of the 

 extinct RhynchoneHidce and in the Spiriferidce, the arms were supported by 

 large spirally-coiled calcareous lamellae (fig. 206). 



Fig. 206. Spit if era hysterica, from the Carboniferous Limestone. The right-hand 

 figure shows the interior of the dorsal valve, with the calcareous spires for the sup- 

 port of the arms. 



DISTRIBUTION OF BRACHIOPODA IN SPACE. All the known 

 Brachiopods live in the sea, and though very local in their 

 distribution, they may be said to have a very wide range. 

 Though sometimes found .between tide - marks, and more 

 commonly in comparatively shallow water, they are essentially 

 deep-water forms, living most generally in depths of from 100 

 to 500 fathoms. A few forms have been found at depths of 

 from 2000 to over 2500 fathoms. About 100 species of living 

 Brachiopods are known. 



DISTRIBUTION OF BRACHIOPODA IN TIME. The Brachio- 

 poda are found from the Cambrian Rocks up to the present 

 day, and present us with an example of a group which appears 

 to be slowly dying out. Nearly four 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 Brachio- 

 poda 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 more 



