In the first of these orders (Inarticulata), 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 Craniadae, Discinidae, and Lingulidae - all very ancient, and all represented at the present day by living forms - together with the Silurian family of the Trimerellidae.
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 Terebratulidae, Rhynchonellidae, and the Thecidiidae, and the extinct families of the Spiriferidae, Pentameridae, Strophomenidae, and Pro-ductidae. 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 Rhynchonellidae and in the Spiriferidae, the arms were supported by large spirally-coiled calcareous lamellae (fig. 206).
Fig. 206. - Spirif era hysterica, from the Carboniferous Limestone. The right-hand figure shows the interior of the dorsal valve, with the calcareous spires for the support of the arms.
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.