In the centre of the folds fossiliferous beds with crinoids have been found, and the black slates at the top of the series contain Myophoria and other fossils, indicating that the rocks are of Triassic age.
Thorpe Cloud, it is highly fossiliferous, but it is usually somewhat barren except for abundant crinoids and smaller organisms. It is polished in large slabs at Ashford, where crinoidal, black and "rosewood" marbles are produced.
Its adhesive foot is paralleled by a cup-shaped ciliated depression, possibly nervous, found in all the larvae cited, except some Echinoderms, and which in Asterids and Crinoids actually serves as an organ of attachment.
The departure of the Ordovician life from that of the Cambrian was perhaps most pronounced in the great development of the molluscs and crinoids (including cystoids), but corals were also abundant for the first time, and graptolites came into prominence.
It was perhaps at this time that the crinoids, as a class, reached their climax, and most forms of lime-carbonate-secreting life seem to have thriven.
- Numerically, the most important inhabitants of the clear Carboniferous seas were the crinoids, corals, Foraminifera and brachiopods.
Corals, both reef-builders and others, flourished in the clearer waters; rugose forms are represented by Amplexoid, Zaphrentid and Cyathophyllid types, and by Lithostrotion and Phillipsastraea; common tabulate forms are Chaetetes, Chladochonus, Michelinia, &c. Amongst the echinoderms crinoids were the most numerous individually, dense submarine thickets of the long-stemmed kinds appear to have flourished in many places where their remains consolidated into thick beds of rock; prominent genera are Cyathocrinus, Woodocrinus, Actinocrinus; sea-urchins, Archaeocidaris, Palaeechinus, &c., were present; while the curious extinct Blastoids, which included the groups of Pentremitidae and Codasteridae, attained their maximum development.
Corals, Archaeocyathus, Spirocyathus, &c., lived in the Cambrian seas along with starfishes (Palaeasterina), Cystideans, Protocystiles, Trochocystites and possibly Crinoids, Dendrocrinus.
The sea-urchins, ophiuroids and crinoids also have yielded many important novelties to A.
Carpenter's proof of the nervous nature of the chambered organ and axial cords of crinoids (Proc. Roy.
The Silurian crinoids and cystids of Sweden have been illustrated in N.
P. Angelin's Iconographia crinoideorum (1878); the Palaeozoic crinoids and cystids of Bohemia are dealt with in J.
Carpenter published important papers on fossil crinoids in the Journal of the Geological Society, on Cystidea in that of the Linnean Society, 1891, and, together with R.
Jaekel, in addition to valuable studies on crinoids and cystids appearing in the Zeitschrift of the German Geological Society, has published the first volume of Die Stammesgeschichte der Pelmatozoen (Berlin, 1899), a richly suggestive work; the Mesozoic Echinoderms of France, Switzerland and Portugal have been made known by P. de Loriol, G.
Abhandl., and Palaeontographica); while the multitude of North American palaeozoic crinoids has been attacked by C. Wachsmuth and F.
Or divisions were based upon certain modifications of structure which, as we now see, affected assemblages of diverse affinity: thus both Blastoidea and Euechinoidea were divided into Regularia and Irregularia; the Holothuroidea into Pneumophora and Apneumona; and Crinoids were discussed under the heads "stalked" and "unstalked."
Some crinoids have yet another circlet below these, the constituent plates of which are called "infrabasals," and are situated radially.
In some crinoids which have no trace of a stem (e.g.
The theory was that all the plates just described, and more particularly those of the cup, which were termed "the calycinal system," could be traced, not merely in all crinoids, but in all Echinoderms, whether fixed forms such as cystids and blastoids, or free forms such as ophiuroids and echinoids, even - with the eye of faith - in holothurians.
Aboral) poles of starfish and crinoids are not comparable with one another, and that all conclusions based on the supposed homology of the dorso-central of echinoids and asteroids with that of crinoids are incorrect.
There is no reason to suppose that the central apical plate of certain free-swimming crinoids has any more to do with the distal foot-plate of the larval Antedon stem than has the so-called centrodorsal of Antedon itself, which is nothing but the compressed proximal end of the stem.
Palaeontology, so far as it goes, is a sure guide, but some of the oldest fossiliferous rocks yield remains of distinctly differentiated crinoids, asteroids and echinoids, so that the problem is not solved merely by collecting fossils.
That the crinoids are all deducible from some such simple form as that above described under the head "calycinal theory," is now generally admitted.
Thus the elements of the Pelmatozoan ventral groove are now detected in so different a structure as the echinoid ambulacrum, while an aboral nervous system, the diminished representative of that in crinoids, has been traced in all Eleutherozoa except Holothurians.
Jaekel (1894) has divided the crinoids into the orders Cladocrinoidea and Pentacrinoidea, the former being the Camerata of Wachsmuth and Springer (Monocyclica Camerata, Adunata and Dicyclica Camerata of the present classification), and the latter comprising all the rest, in which the arms are either free or only loosely incorporated in the dorsal cup. In minor points there is fair agreement between the American, German and British authors.