Crinoidea are very rare in a living state, Echinoidea plentiful. Now 29 species of Crinoidea occur in the Lower Paleozoic, 15 in Middle Palæozoic, and 105 in Upper Paleozoic 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 Paleozoic series by 12 species, in the Mesozoic by 173, and in the Cenozoic by 25.

All the Echinoidea of the Paleozoic 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 Paleozoic Series have such peculiarities in the series of plates and pores as to claim to be enrolled in separate families (Palæchinidæ and Archæocidarida), or even to constitute an order (Perischoechinoidea) equal to that of the Echinoidea.

The contrasts which have thus been stated between the Paleozoic and later forms of Radiated Animals may be represented in the following tabular view where the word Neozoic is used to include Mesozoic and Cenozoic ages.

Annelida.-In the modern seas we find two great groups, Cephalobranchiate and Dorsibranchiate Annelida. The same two orders exist in the Lower Paleozoic 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 Paleozoic periods; the other (Malacostraca) not yet certainly discovered therein.

Among the Entomostraca the large groups of Trilobitidæ and Eurypteridæ are peculiar to the Paleozoic 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 Reteporidæ, which are almost confined to Palæo

Trilobitidæ.
Eurypterida.

Entomostraca.

CRUSTACEA.

(Entomostraca.
Malacostraca.

zoic Strata, their families are pretty equally dif fused.

Tunicata, another somewhat less abnormal group, not covered by shell, is not recognized in a fossil state.

Brachiopoda abound in the Strata, and are scattered through the modern Seas in somewhat greater numbers than was formerly supposed, when dredging at considerable depths was not practised. They are rare in the Cenozoic Strata. Of nearly forty genera and subgenera upwards of twenty appear limited to the Paleozoic Series; only two are mentioned among the Cenozoic fossils of Britain; but twelve at least occur in various modern oceans. The families of Spiriferida, Orthidae, and Productidæ are confined to the Palæozoic Strata; Terebratulidæ, Rhynchonellidæ, Craniada, Discinidæ, and Lingulidæ may be regarded as of all periods; Lingula, Discina, Crania, Rhynchonella, Thecidium, Argiope, Terebratella, Waldheimia, Terebratulina, Terebratula, are both fossil and recent. Of these, Terebratula, Rhynchonella, Crania, Discina, Lingula, pass through all the great periods of Geology and still exist, with peculiarities of structure and association very like those which belonged to them in the earliest periods. For example, the Lingulæ of every age always shew almost exactly equal, delicate, depressed, nearly