genera of the Palæozoic systems belong to this division; the same is true for the Mesozoic series; and even in the Cænozoic strata Alcyonoid Zoophyta are rare. Here, then, is one great order of Zoophyta continuous through the whole series. The genera change with the successive deposits, but there is one remarkable law of structure which is characteristic of period. The radiating plates of the Coral are in a young state pretty regular in number and in the mode of division. In all the Palæozoic Strata the primary or principal plates are four, or some multiple of four; in all the Mesozoic and Cænozoic Strata they are six, or some multiple of six. This striking generalization, due to Milne Edwards and the late Jules Haime, is thought to be subject to no more than solitary exceptions. It suggests the reflection that the persistence of characters which we observe in modern living nature was quite as remarkable in ancient organizations, and throws a heavy weight into the scale against the doctrine of the later forms of life being derived from earlier types, through natural variations integrated by time. Echinodermata. - Six fossil groups represent this beautiful class of animals, viz. Crinoidea, Blastoidea, Cystoidea, Ophiuroidea, Asteroidea, Echinoidea. Of these, two are only known fossil (Blastoidea and Cystoidea), and they belong to the Palæozoic Strata. Crinoidea are very rare in a living state, Echinoidea plentiful. Now 29 species of Crinoidea occur in the Lower Palæozoic, 15 in Middle Palæozoic, and 105 in Upper Palæozoic Strata. After this they grow comparatively rare, though still 29 species occur in Mesozoic Strata, and 5 in the Cenozoic series. One recent species! On the other hand, Echinoidea are represented in the Palæozoic series by 12 species, in the Mesozoic by 173, and in the Cænozoic by 25. All the Echinoidea of the Palæozoic series belong to the regular division, with the openings of the alimentary canal opposite (E. endocyclida). The same group occurs in the Mesozoic Series, but in addition we have a second equally large group with these openings not opposite (E. exocyclida), and this is continued in the modern ocean. The Echinoidea of the Palæozoic Series have such peculiarities in the series of plates and pores as to claim to be enrolled in separate families (Palæchinide and Archæocidaride), or even to constitute an order (Perischoechinoidea) equal to that of the Echinoidea. The contrasts which have thus been stated between the Palæozoic and later forms of Radiated Animals may be represented in the following tabular view; where the word Neozoic is used to include Mesozoic and Cænozoic ages. Annelida. In the modern seas we find two great groups, Cephalobranchiate and Dorsibranchiate Annelida. The same two orders exist in the Lower Palæozoic Strata, sometimes containing species of very large dimensions, and they seem to have been, as now, mostly prevalent on muddy shores. Cirripedia. These beautiful animals are not recognized in Palæozoic Strata. Crustacea. Dividing them into two groups we find one (Entomostraca) extremely prevalent in the Palæozoic periods; the other (Malacostraca) not yet certainly discovered therein. Among the Entomostraca the large groups of Trilobitidæ and Eurypteridæ are peculiar to the Palæozoic ages. The relations to time which thus appear among the Articulata are represented in the following Table. Polyzoa, or Bryozoa. These abnormal, often compound mollusks, in their mode of growth and membranous or stony parts much resembling Zoophyta, occur in all the groups of Strata, and excepting Reteporide, which are almost confined to Palæo { Trilobitidæ. Eurypteridæ. Entomostraca. CRUSTACEA. (Entomostraca. |