The Metazoa offer similar examples of persistence. Among the Cœlenterata, we have the genus _Springopora_, whose representatives from the Carboniferous limestones closely resemble some of the present-day reef builders of the East Indies. Species of the brachiopod genera _Lingula_ and _Crania_ occurring in the Cambrian rocks are indistinguishable from species living today, while two other modern genera of the Brachiopoda, namely, _Rhynchonella_ and _Discina_, are represented among the fossils found in Mesozoic formations. _Terebratulina striata_, a fossil species of brachiopod occurring in the rocks belonging to the Cretaceous system, is identical with our modern species _Terebratulina caput serpentis_. Among the Mollusca such genera as _Arca_, _Nucula_, _Lucina_, _Astarte_, and _Nautilus_ have had a continuous existence since the Silurian, while the genera _Lima_ and _Pecten_ can be traced to the Permian. One genus _Pleurotomaria_ goes back to pre-Cambrian times. As to Tertiary fossils, Woods informs us that “in some of the later Cainozoic formations as many as 90 per cent of the species of mollusks are still living.” (“Palæontology,” 1st ed., p. 2.) Among the Echinodermata, two genera, _Cidaris_ (a sea urchin) and _Pentacrinus_ (a crinoid) may be mentioned as being persistent since the Triassic (“oldest” system of the Mesozoic group). Among the Arthropoda, the horseshoe crab _Limulus polyphemus_ has had a continuous existence since the Lias (_i.e._ the lowest series of the Jurassic system). Even among the Vertebrata we have instances of persistence. The extant Australian genus _Ceratodus_, a Dipnoan, has been in existence since the Triassic. Among the fossils of the Jurassic (middle system of the Mesozoic group), _Sharks_, _Rays_, and _Chimaeroids_ occur in practically modern forms, while some of the so-called “ganoids” are extremely similar to our present sturgeons and gar pikes—“Some of the Jurassic fishes approximate the teleosts so closely that it seems arbitrary to call them ganoids.” (Scott.) The instances of persistence enumerated above are those acknowledged by evolutionary palæontologists themselves. This list could be extended somewhat by the addition of several other examples, but even so, it would still be small and insufficient to tip the scales decisively in favor of fixism. On the other hand, we must not forget that the paucity of this list is due in large measure to the fact that our present method of classifying fossiliferous strata was deliberately framed with a view to excluding formations containing modern types from the category of “ancient” beds. Moreover, orthodox palæontology has minimized the facts of persistence to an extent unwarranted even by its own premises. As the following considerations indicate, the actual number of persistent types is far greater, even according to the evolutionary time-scale, than the figure commonly assigned.