These curious, and in the modern seas, exceptional shells, were dominant in the Palæozoic period. Upwards of three thousand fossil species are known, of which a large proportion belong to the Cambrian and Silurian, nine genera appearing in the Cambrian, and no less than fifty-two in the Silurian. The history of these creatures is very remarkable. The Lingulæ, which are the first to appear, continue unchanged and with the same phosphatic shells to the present day. Morse, who has carefully studied an American species, remarks in illustration of this, that it is exceedingly tenacious of life, bearing much change of depth, temperature, etc., without being destroyed. The genus _Discina_, which is nearly as old, also continues throughout geological time. The genus _Orthis_ (Fig. 57), which appears at the same time with the last, becomes vastly abundant in Silurian times, but dies out altogether before the end of the Palæozoic. _Rhynchonella_ (Fig. 58), which comes in a little later, near the beginning of the Siluro-Cambrian, continues to this day. _Spirifer_ and _Productus_ (Figs. 59 and 60) appear later, and die out at the close of the Palæozoic. So strange and inscrutable are the fortunes of these animals, which on the whole have lost in the battle of life, that their place in nature is vastly less important than it was. It has been suggested that if any group of creatures could throw light upon the theory of descent with modification, it would be these; but Davidson, who has perhaps studied them more thoroughly than any other naturalist, found them as silent on the subject as the sponges or the corals. In a series of papers published in the _Geological Magazine_, a short time before his death, he remarked as follows: [Illustration: FIG. 56.—_Terebratula sacculus_ (Martin). Carboniferous.] [Illustration: FIG. 57.—Brachiopods; genus _Orthis_. _a_, _O. Billingsi_ (Hartt). Lower Cambrian. _b_, _O. pectinella_ (Hall). Siluro-Cambrian. _c_, _O. lynx_ (Eichwald). Siluro-Cambrian.] [Illustration: FIG. 58.—_Rhynchonella increbrescens_ (Hall). Siluro-Cambrian.] [Illustration: FIG. 59.—_Spirifer mucronatus_ (Conrad). Devonian.] [Illustration: FIG. 59_a_.—_Athyris subtilita_ (Hall). Carboniferous. _a_, _b_, Exteriors. _c_, Interior, showing spirals.]