BRACHIOPODA.
The lamp-shells (Brachiopods), more than any other group, have suffered with the lapse of time. Of 1300 known species, only 75 are living ; and of the 34 genera, the larger part (21) are extinct. The number of generic forms is greatest in the Devonian period and least in the upper oolites, after which a second set of new types gradually appears. The preponder ance of fossil Brachiopoda is contrasted with the scarcity of the recent shells even more strongly by the abundance of indi viduals than by the number of species ; for the living shells mostly inhabit deep water and rocky situations inaccessible to the dredger, and are seldom obtained in large numbers.
The genus Terebratula, as now restricted to shells with a short internal loop, musters above 100 fossil species, of which only one survives (T. vitrea), an inhabitant of the Lusitanian province. The Waldheimias, or Tere,bratuke with long loops, are widely distributed in our present seas, although only nine living species are knov. n ; individuals of one or more of these are found on the coast of Spitzbergen and Labrador, at Cape Horn, and most abundantly in New South Wales and New Zealand : there are sixty fossil species dating from the trial. The Terebratelke, having the loop fixed to a mid ridge, com menced in the lias, and occur in small numbers throughout the cretaceous and tertiary periods, and are the only lamp-shells which attain their climax in recent seas. Five species of Argiope occur in the greensand, chalk, and tertiaries. The allied genus Thecidium is represented by one species in the carboniferous and one in the triassic systhm, becomes comparatively common in the secondary period, and dwindles again to a single species in the newer tertiary ; this species survives within still narrower limits in the Mediterranean Sea. The sub-genus Tere,bratu lina is represented by twenty species in the secondary and Brachiopods.
tertiary formations. 2'. striata of the chalk is so like the recent T. caput serpentis as to be with difficulty distinguished from it.
Several extinct sub-genera occur in the cretaceous strata, of which the most remarkable are Trigonosemus (fig. 11, i) and Lyra, shaped like a violin. The genus Stringocephalus (fig. 11, 2) is peculiar to the Devonian strata, and has a large internal loop, and a very prominent cardinal process, forked at the end, and fitting over the central plate of the opposite valve.
The shell of Tere,bratula and some of its allies (Argiope, Thecidium, Cyrtia, and Spiriferina) is dotted with minute quincuncial perforations, sometimes visible to the naked eye, as in T. lima, but usually requiring a lens of low power. They are smallest in T. carnea.
The lamp-shells with sharp beaks and plaited valves have been separated from the Terebratulx under the name Rhyn chaaella (Fisch.) Their shells do not exhibit the punctate structure under a magnifying-glass, and they have no internal skeleton to support their arms, which in the recent species are coiled up spirally, and directed towards the concavity of the smaller valve, like the spires of the extinct Atrypa (fig. 11, 7). Of the three living species of Rhynchonella, one is found throughout the Arctic Seas, a second in New Zealand, and the third at the Feejees (1). The fossil species exceed 250, and are found in all parts of the world ; those from the palteozoic strata may prove distinct from the rest, since the permian species are known to be provided with large internal processes (Camarophoria, King). Casts of these shells are frequently impressed with the narrow and angular pallio-vascular impres sions. The extinct genus Atrypa differs from Rhynchonella solely in having calcareous spires, which are preserved in many instances, and may be cleared to some extent by the application of acid. The foramen is separated from the hinge line by a deltidium ; and the interior of the valve is marked by ovarian and vascular spaces exactly as in Rhynchonella. The lower Silurian rock contains another genus, Porambcmites (Pander), as yet imperfectly understood, but having the valves marked externally by impressed dots, which are not perfora tions. The genus Pentamerus occurs in all the strata below the carboniferous limestone, and is remarkable for its great internal partitions, causing the shell to split readily across the middle ; and giving rise to deep incisions in those casts of the interior which are so common in the Caradoc sandstone (fig. 11, s).