Fig. 59.—Spirifer mucronatus (Conrad). Devonian.
Fig. 59a.—Athyris subtilita (Hall). Carboniferous.
a, b, Exteriors. c, Interior, showing spirals.
“We find that the large number of genera made their first appearance during the Palæozoic periods, and since they have been decreasing in number to the present period. We will leave out of question the species, for they vary so little that it is often very difficult to trace really good distinctive characters between them; it is different with the genera, as they are, or should be, founded on much greater and more permanent distinctions. Thus, for example, the family Spiriferidæ includes genera which are all characterised by a calcified spiral lamina for the support of the brachial appendages; and, however varied these may be, they always retain the distinctive characters of the group from their first appearance to their extinction. The Brachiopodist labours under the difficulties of not being able to determine what are the simplest, or which are the highest families into which either of the two great groups of his favourite class is divided; so far, then, he is unable to point out any evidence favouring progressive development in it. But, confining himself to species, he sees often before him great varietal changes, so much so as to make it difficult for him to define the species; and it leads him to the belief that such groups were not of independent origin, as was universally thought before Darwin published his great work on the Origin of Species. But in this respect the Brachiopoda reveal nothing more than other groups of the organic kingdoms.
Fig. 60.—Productus cora (D’Orbigny). Carboniferous.