GEOLOGICAL DISTRIBUTION OF CRINOIDEA.
Geological distribution of the Crinoidea.—The great number of extinct forms of this order of Radiata in the most ancient fossiliferous deposits, is a remarkable fact, which has already been incidentally adverted to. In the palæozoic seas—including the Silurian, Devonian, Carboniferous, and Permian—the Crinoidea were represented by upwards of fifty genera, whose existence began and ended during that geological cycle.
According to the present state of our knowledge all those peculiar types of radiated animals were created, and each lived through the destined period alloted to its race, and died out ere the deposition of the New Red Sandstone; not a single species, not a relic of the innumerable individuals that swarmed in the palæozoic oceans, has been observed in any strata above the Permian.
The Trias, which ranks as the earliest of the secondary formations, is characterized by the advent of two typical genera; the true Encrinus or Lily-encrinite, and the Pentacrinus; the former is unknown in any other deposits; the duration of its race was comprised within the Triassic epoch. The Pentacrinus, on the other hand, has been perpetuated through all the succeeding periods, and one species inhabits the present seas; the sole existing representative of the most ancient type of this order.
In the Oolite, another living form, the Comatula, first appears.
The ocean of the Cretaceous epoch was inhabited by five genera of Crinoids, unknown elsewhere; among them is that remarkable genus, the Marsupite.
The Crinoidea of the Tertiary seas are as few in number and variety as those of the present day; not a vestige of any of the ancient tribes has been discovered. M. D'Orbigny's Tab. 12 presents the phenomena thus briefly noticed, in a striking point of view.
From this review of the fossil Crinoidea and Asteriadæ, the student will be in some measure prepared for the collecting of instructive specimens from the immense accumulation of remains imbedded in certain strata of the Oolitic, Liassic, Carboniferous and Silurian rocks.