The Senonian consists of a central facies with Micraster peini; a meridional facies with Ostrea; and a northern facies developed round Tunisia with large forms of Inoceramus and echinoids.
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.
The theory has been vigorously opposed, notably by Semon (op. cit.), who saw in the holothurians a nearer approach to the ancestral form than was furnished by any calyculate echinoderm, and by the Sarasins, who derived the echinoids from the holothurians through forms with flexible tests (Echinothuridae, which, however, are now known to be specialized in this respect).
The support that appeared to be given to the theory by the presence of supposed calycinal plates in the embryo of echinoids and asteroids has been, in the opinion of many, undermined by E.
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.
Those plates are perhaps constant throughout sea-urchins and starfish (though it would puzzle any one to detect them in certain Silurian echinoids), and they may be traced in some of the fixed echinoderms; but there is no proof that they represent the radials of a simple crinoid, and there are certainly many cystids in which no such plates existed.
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.
In more typical echinoids the covering-plates alone remained to form the ordinary ambulacral plates, while the flooring-plates disappeared, the canals and other organs remaining as before.
- Denuded tests of some fossil Echinoids.
On the foreshore during scouring tides, the chalk yields echinoids and sponges.
The irregular echinoids are, however, quite a different story.
Other fossils include the sea urchins (echinoids ), which occur in a number of varieties.
Every one knows that an "apical system," composed of two circlets known as "genitals" or basals and "oculars" or radials, occurs round the aboral pole of echinoids (fig.
The echinoids or sea-archins (see SEA-Urchin) may be grouped under the following orders, here named in the sequence of their appearance in the rocks.
Other fossils include the sea urchins (echinoids), which occur in a number of varieties.
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